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KENYATTA UNIVERSITY INSTITUTE OF OPEN DISTANCE & e-LEARNING IN COLLABORATION WITH SCHOOL E.G. SCHOOL OF PURE & APPLIED SCIENCES DEPARTMENT OF BOTANY SBT 102 PLANT MORPHOLOGY AND ANATOMY WRITTEN BY: COURSE AUTHOR’S NAME EDITED BY: COURSE AUTHOR’S NAME Copyright © Kenyatta University, 2009 All Rights Reserved Published By: KENYATTA UNIVERSITY PRESS TABLE OF CONTENTS LECTURE ONE INTRODUCTION TO THE STUDY OF PLANT MORPHOLOGY AND ANATOMY 1.0 Introduction We welcome you to the study of morphology and anatomy. This unit is one of the core units in the field of botany. The study will mainly deal with the group of plants called the vascular plants or tracheophytes. In this we shall look at both the external morphology and internal anatomy of representative organs of plants. 1.1 Objectives By the end of this lesson you should be able to: Define the term “plant morphology” Distinguish the groups of plants, which comprise the tracheophytes or vascular plants. Distinguish between a cross section (or transverse section) from a longitudinal section of a plant organ such as a stem or root. Identify the anatomical and morphological features that distinguish the angiosperms from the pteridophytes and gymnosperms. 1.2 content 1.2.1 The meaning of Plant Morphology and Anatomy The term plant morphology refers to the form of plants. This is with r4egard to the different plant organs, such as sterns, leaves, roots and flowers. The term plant anatomy is particular used to refer to the internal structure of plant organs, such as stems, roots, leaves and flowers. To be able to study the internal anatomy of a plant organ you will require to cut cross sections (or transverse sections) of the particular organ. Some times it is also necessary to cut longitudinal sections of the plant organ. Such sections are then viewed or examined under the light compound microscope in order to see the distribution of the various cell types and tissues. Permanent stained slides may also be used. When examining a specimen under a microscope please start by using the low power of the microscope. Under this power you should be able to see the general layout or distribution of tissues. To see cell details you have to switch to the medium power. Generally the use of the high power should be done under adequate instruction. 1.2.2 A classification of vascular plants (or Tracheophytes) As indicated earlier in the lesson we shall deal mainly with the morphology and anatomy of the vascular plants. In view of his it will be useful to familiarize yourselves with the classification of the major group of plants. (a) Non seed plants Pteridophytes The classification include the sub divisions: (i) (PSILOPSIDA (spore bearing) Mainly occurring in fossil form but a few living genera are found e.g. Psilotum. (ii) LYCOPSIDA – tracheid and spore bearing Including living genera, such as: Lycopodium – Clubmosses e.g. Lycopodium clavatum Phylloglossum Selaginella. (iii) SPHENOPSIDA Largely extinct, with only one living genus Equisetum. (iv) PTEROPSIDA This is the largest subdivision of the vascular plant (Tracheophytes). It includes: a) The ferns (Filicinae) - Spore producing (non seed) - With tracheids. b) Seed plants – Phanerogams Gymnosperms Seed borne on scales (naked seeds) and having tracheids in the xylem. Angiosperm or flowering plants. Seeds borne inside a fruit (fruit wall) and having xylem vessels. Angiosperms are 2 kinds: Monocotyledonous plants e.g. maize, grass etc. Characterized by having: - One cotyledon Scattered vascular bundles in the stem. Dicotyledonous plant e.g. Bean, sunflower etc. Characterized by having 2 cotyledons Vascular bundles arranged in a ring in the stem. In the evolution of the plants it has been suggested that there was a gradual movement from water onto land. This necessited the development of a vascular system of the tracheids and xylem vessels. This system provides channels for the conduction of water and dissolved solutes from the soil to the leaves. This development led to the 3 groups of plants that may be listed as: 1. Pteridophytes 2. Gymnosperms 3. Angiosperms referred to above 1.3 Questions 1. Outline the classification of the vascular plants. 2. What special anatomical development helped the movement of plants from water onto land? 1.3.3. The Morphology and anatomy of Lycopodium clavatum Morphology Lycopodium species including L. clavatum, grow as creepers on the ground. The creeping stem bears leaves and at intervals produces vertical shoots to form strobili (or cones). Each fertile leaf in a strobilus bears a single large sporangium containing spores. The shoot of Lycopodium clavatum is dichotomously branched. IMAGE FIG 1.0) a diagram of Lycopodium calavatum, showing leaves and rhizoids and dichotomous branching Anatomy A transverse section of the stem of Lycopodium clavatum, as shown in figure 1.1. shows the following features: - A wide layer of cortex. - An endodermis encompassing the vascular system. - A vascular system consisting mainly of xylem and phloem. - The xylem is made up of tracheids, Fig. 1.1 IMAGE FIG. 1.1 Diagram of transverse section of Lycopodium stem/rhizome showing the distribution of tissues. It is to be noted that the vascular systems is at the center, with phloem being found in between army of xylem. 1.3.4 Morphology AND ANATOMY OF PTERIDIUM AQUILINUM We shall here briefly look at the morphology and anatomy of the more common fern, bracken fern (Pteridium aquilinum) In Kenya this fern commonly found growing on red acidic soil of the highlands. It is a persistent weed It is to be noted that the sporophyte i.e. the spore-producing plant is the conspicuous plant. This plant has its stem growing Underground in the form of a rhizome, which produces roots and aerial shoots at intervals, as shown in Fig, 1.2 IMAGE 1.2 Diagram of Bracken fern (Pteridium aquilinum) consisting of the sporophyte generation with a rhizome, aerial shoot or frond bearing sori on the under side that contain spores. The rhizome bears roots. The mature plant produces spores contained in sori Fig. 1.3. Such sori are borne on the underside of the leaves fronds Fig. 1.2. The spore bearing leaves are known as the sporophylls. IMAGE Fig. 1.3 A cross section of a single sorus of Bracken fern containing sporangia on the under side of the leaf. The sorus has a protective shield called the indusium. A transverse section of the rhizome of Pteridium aquilinum, Fig. 1.4 (a) shows Dictyostele. This is a system in which there are separate vascular bundles. The xylem here is made up of tracheids, Fig. 1.4 (b). IMAGE Fig. 1.4 (a) Diagram of a transuerse section of stem/rhizome of Bracken fern (Pteridium aquilinum) showing inner and outer vascular bundles with intervening sclerenchyma the xylem consists of tracheids. Characteristics Features of Tracheids (i) Tracheids are dead cell elements (ii) The walls of tracheids are very thick (iii) The walls have pits (iv) Tracheids have tapering ends, fig. 1.4 (b) IMAGE Fig 1.4 (b) A longitudinal section of several tracheids showing their characteristic features of thick walls, tapering ends and possession of pits. Tracheids strengthen the plants. 1.3.5 The life cycle of the Bracken fern (Pteridium aquilinum) as noted before the vegetative plant is the sporophyte, which bears/ produces spores. When the spores are shed from the sori and fall on warm moist soil they germinate. A colour less rhizoid grows which serves to absorb water and mineral salts from the soil. Through repeated cell division a prothallus is formed Fig. 1.5. IMAGE Fig. 1.5 Diagram of a fern prothallus underside showing male and female organs and rhizoids. The prothallus develops into a heart shaped structure. The growing point is located at the notch end. On the lower surface of the prothallus rhizoids are formed towards the tapering end. The reproductive structure/organs are found on the lower surface of the prothallus. These consist of several female organs, archegonia at the notch end and the male organs i.e., antheridia located amongst the rhizoids at the tapering end, Fig. 1.5. Each archegonium contains an egg (ovum). The antheridia produce antherozoids which swim in water and reach the archegonia. One of the antherozoids unites with the egg in the archegonium resulting in a zygote, which is diploid. Through repeated cell division of the zygote an embryo is formed which develops a root and stem/shoot and a frond (leaf). As a result a sporophyte plant is established, Fig. 1.6. IMAGE Fig. 1.5 Diagram of a fern prothallus underside showing male and female organs and rhizoids. Activity 1. Make a labeled drawing of the transverse section of the rhizome of the bracken fern (pteridium aquilinum). 2. Give a well illustrated account of the anatomical features of a tracheid. 3. Collect two or three type types of pteridophytes and dry them by pressing them between newspaper sheets mount them, and label the morphogical features. Summary You will now have realized that morphology and anatomy is a basic field of study for a botanist. It provides information on the structure of the plant. We have noted that the vascular plants have structures that strengthens them as well as providing a system for conduction of water and solutes from the soil to the leaves. This is due to elements such as tracheids and xylem vessels. [it has also been noted that the sporophyte plant is the more prominent phase while the gametophyte much smaller in size. The sporophyte is haploid (n) while the gametophyte is diploid (2n)]. Definition of key words Morphology – refers to form Tracheophytes or vascular plants – plants with vascular elements such as tracheids and vessels of the xylem. Pteridophyte – first group of truelly terrestrial plants e.g. fern. Gymnosperm – group of seed plants whose seeds are not coloured by any structure (naked). Angiosperm – group of plants whose seeds are contained in fruit. Sporophyte – vegetative plant bearing spores. Gametophyte – plant producing gametes Strobilus – a collection of leaves called sporophylls bearing sporangia. Sporangium – a structure containing spores Sorus – a structure, which houses spores and found in pteridophytes. Prothallus – a heart shaped structure bearing both archegonia and antheridia on the underside. It constitutes the gametophyte of a pteridophyte. Gametophyte – gamete bearing phase of a plant. Gamete – male and female cells which unite during fertilization, e.g. Egg or ovum and antherozoid. Pit – an opening in the wall of a tracheid or vessel. Gametophyte – a gamete producing plant that is generally haploid and produce the gametes by mitosis. REFERENCES Esau, K. 1960. Anatomy of seed plants. Esau, K. Plant Anatomy Fahn, A. 1974 Plant Anatomy Foster, A. S. Comparative Morphology of Vascular plants. Fuller, H.J. and O. Tippo. 1949, College Botany. Sporne, K.R. The Morphology of Angiosperms. The structure and evaluation of flowering plants. Steeves, T.A. 1989, Patterns in plant development. LECTURE TWO THE EXTERNAL MORPHOLOGY OF THE GYMNOSPERMS AND THE ANGIOSPERMS. 2.1 INTRODUCTION In lesson 1, we looked at the introduction to the study of Plant Morphology and Anatomy. In unit 2 or lesson 2, we are going to describe the external morphology of the gymnosperms and the angiosperms. These are the seed bearing plants. The study will include the shoot system and root system. Differences between the external morphology and the organs of the gymnosperms will be highlighted. 2.2 OBJECTIVES By the end of this lesson, you should be able to: Describe the external morphology of stem, branch, leaf and bud of an angiosperm (monocotyledon and dicotyledon) Describe the external morphology of the root of an angiosperm. Distinguish the external morphology of the leaves of gymnosperms from those of angiosperms. Describe the external morphology of the specialized stems, such as the rhizomes, stem tubers and bulbs. Identify the different types of roots, such as, the taproots, secondary roots, fibrous roots, aerial roots and proproots. Draw the external morphology of specimens during practical work sessions. 2.3 A STUDY OF EXTERNAL MORPHOLOGY OF SEED BEARING PLANTS (ANGIOSPERMS AND GYMNOSPERMS) In this study we would like to start by looking at the more familiar plants and later proceed to the less familiar ones. In view of this we suggest that we first look at the external morphology of the Angiosperm plants (flowering Plants). These comprise of the monocotyledonous and dicotyledonous plants. 2.3.1 the external Morphology of the angiosperm plant and its organs. The development of the embryo into a seeding. Following fertilization which in an ovule is through the union of an egg (ovum) and the male gamete, a zygote is formed. Through repeated cell division of the zygote a young embryo is formed. In the case of dicotyledonous plant, such an embryo may be illustrated in an opened soaked bean seed Fig. 2.1. In the practical session you should note, draw and label the following features: Testa or seed coat Two cotyledons The embryo held between the two cotyledons The embryo consists of (i) 2 plumule leaves (ii) an axis (or rudimentary stem) and (iii) a radicle IMAGE Fig. 2.1 (a) The structure of a soaked opened up bean seed showing a young embryo two cotyledons and a seed coat or testa. Under suitable conditions of adequate moisture, oxygen concentration and temperature the bean seed will germinate. The radicle will protrude and grow down wards while the two plumule leaves and the axis will grow upwards to form the hypocotyls and epicotyl of the young seedling. The radicle will give rise to the root system consisting of a tap root and secondary roots, as shown in Fig. 2.1 (b). IMAGE Fig. 2.1 (b) A young bean seedling showing 2 foliage leaves, 2 cotyledons, a shoot apex, an epicotyl, a hypocotyls, tap root apex and secondary roots (lateral roots). In the practical session you should draw and label the various features of a dicotyledonous seedling. The young seedling eventually grows into a mature vegetative plant. 2.3.1.2 The shoot system Such a mature plant will have a shoot system comprising of : The stem with nodes, internodes dormant axillary buds, apical or terminal bud. Lateral branches. Foliage leaves, as shown in Fig. 2.1 (c) By virtual of processing of chlorophylls the shoot carries out the all important function of photosynthesis and therefore production of food for the plant. IMAGE Fig. 2.1 (c) The external morphology of a dicotyledonous flowering plant showing the main organs: stem, nodes, internodes, foliage leaves, axillary buds, shoot apex, a lateral branch, a flowering shoot, tap root, lateral roots and root apex. Specially modified organs of the shoot system, such as the stem and leaves may perform special functions as explained below. 2.3.1.2.1 Specialized stems We shall describe to you some examples of specialized stems as follows: 2.3.1.2 The stem tubers A good example of stem tubers are the Irish/English potato plant (Solanum tuberosum) tubers which store large quantities starch. This is an important crop. The Irish potato tuber is identified as an underground stem. In the practical session you should make drawings of Irish potato tuber to show the following features, as illustrated in Fig. 2.2 (a) Tiny, leaf scales Eyes or dormant axillary buds IMAGE Fig.2.2 (a) An Irish potato tuber (Solanum tuberosum) showing ‘eyes’ or dormant auxiliary buds and tiny leaf scales. Other forms of underground stems are rhizomes bulbs and corns. These also store food stuffs. Bulbs e.g. Onions The bulb of the onion consists of a small stem which bears many fleshy leaves. Axillary buds develop in the axils of the leaves as shown in fig.2.2. IMAGE Fig. 2.2 (b) A longitudinal section (L.S) of an onion bulb showing the compact underground stem, fleshy leaves an adventitious roots. 2.3.1.2 Structures on the surface of stems Various structures may occur on the stem as outlined below: Hairs – the stem surface of a herbaceous plant may bear hairs, which are outgrowths of epidermal cells. Stipules: which are small projections at the junction of leaf petiole with the stem. Spines: which may be modified twigs or leaves. Woody stems and twigs of trees and shrubs have structures as follows: Lenticels: which are tiny pores/openings which facilitate an exchange of gases between inner tissues of the stem and outside atmospheric air. Leaf scars: which are the marks left when leaf stalks fall off from the stem or twig. Bundle scars: which are the broken ends of the vascular bundles. Bark; as the woody stems grow and increase in diameter (girth) the smooth outer bark splits. As a result the surface of the stem becomes rough, old bark peel off in the course of time. 2.3.1.2.3 FOLIAGE LEAVES Leaves of dicotyledonous plants Types of leaves There are three types of leaves, as follows: Simple leaves Compound leaves, and Complex leaves Simple leaves A typical simple leaf of a dicotyledonous plant consists of a leaf blade and a leaf stalk (Petiole). In some plants the simple leaf may not have a leaf stalk in which case it is said to be sessile leaf. The point of attachment of the leaf to the stem is called the node as shown in Fig. 2.3 (a). some leaves have stipules which are small projections at the juncture of the petiole and the stem. IMAGE Fig. 2.3 (a) Showing a simple leaf with an entire margin A simple leaf may have a serrated or lobed leaf blade, as in Fig. 2.3 (b) and Fig. 2.3 (c) respectively. IMAGE Fig. 2.3 (b) Showing a simple leaf with a serrated margin e.g. Hibiscus sp. IMAGE Fig. 2.3 (c) Showing a simple palmately lobed leaf e.g. Trimfeta sp. The plastochron The plastochron is the period between the successive initiation of leaves or two pairs of leaves on the stem. Modified leaves for special functions. There are several types of modified leaves as follows: Spines and thorns In the cactuses the leaves have become modified into thorns or spines. This is an adaptation for the reduction of the rate of transpiration. Water storage leaves Certain plants growing in the arid and semi-arid areas, such as the Aloe have thick leaves containing mucilaginous colloidal materials which hold water firmly. Tendrils In some plants some of the leaves are modified into tendrils which are used for climbing, e.g. the garden pea. Leaves of insectivorous/carnivorous plants. Examples: The pitcher plant The Venus fly – trap plant The sundew (Drosera rotundifolia) Other leaf types - Succulent leaves – thick and fleshy - Sclerophyllous foliage leaves Produce more sugars by photosynthesis than are used in their own construction - Soft, flexible and edible. - Thorns, prickles, spines – in cacti are modified leaves protect the plant. - Tendrils – leaves modified partly or completely as tendrils. Used to help the plant in climbing by curling tightly around an object to support weak stems. - Reproductive leaves e.g.bryphyllum Plantlets sprout on edge of leaves. Floral leaves (bracts) specialized leaves found at the basis of flowers or flower stacks e.g. Poisentia – brightly coloured floral bracts around the small flowers. Uses of leaves - Shade - Food – cabbages - Spices – thyme, oregano - Dyes – red from henna - Cordage fibres for ropes ogave (sisal) hats, bags, thatching huts – makuti. - Oils – essential voldutile oils - Medicinal e.g cocaine as anesthetic - Tobacco - Intoxicants – marijuana, miraa - Beverages – tea. These plants which grow in swampy habitats become deficient of certain essential elements. They use specialized mechanism to trap small insects which they digest in order to obtain the deficient elements or nutrients. THE LEAVES OF MONOCOTYLEDONOUS PLANTS These are characterized by having: A leaf sheath instead of a leaf stalk (petiole). Parallel venation in the leaf blade as shown in Fig. 2.4. IMAGE Fig. 2.4 A portion of the stem of the maize of the maize plant showing two nodes, a leaf shealth around the internodes and parallel vein leaf. Roots The roots of a plant collectively form the root system. Types of Roots The Tap Root system: dicotyledonous particularly/trees, shrub and herbs have a tap root system, which bear secondary roots. The Fibbrous Root system: fibrous roots are particularly common. Monocotyledonous plants, particularly the grass family plants (cereals), These roots are numerous and grow shallowly in the soil, see Fig. IMAGE Fig.2.5: Showing fibrous roots of a grass plant. Functions of roots There are two main functions of roots. 1. The anchorage of the plant in the soil, and 2. The absorption of water and dissolved minerals salts. 2.3.1.3 The various regions of a growing root A growing root is charactersized by having the following regions: (i) The Root Cap. This is an apical cap-like tissue whose main functions is to protect the delicate meristematic region immediately behind it. the root cap produces new cells to replace the cell worn out or destroyed as the root tip is bruised by rocks and soils particle. Position of the rock cap is shown in Fig. 2.6. IMAGE Fig. 2.6 Diagram of a root showing the various regions (ii) The Root Apical Meristem This region is enclosed by the root cap. It is a region in which new cells are produced through division of the meristematic cells. (iii) The Region of Cell Elongation This is the region behind the root apical mersitem. In this region all the cells greatly elongate. (iv) The Root hair region This is a region noted for bearing root hairs. These are epidermal cells which grow laterally and serve for the absorption of water and dissolved mineral salts. (v) The mature region In this region the cells are mature and have undergone differentiation to carry out various functions and no longer elongate. Table 2. External differences between stems and roots of angiosperms Stems Roots 1. Stem of most plants are Roots are positively geotropic negatively geotropic (gravitopic) grow upward 2. Stems have well developed nodes and internodes Roots do not have nodes and internodes 3. Branches of stems arise externally Root branches or secondary roots from axillary buds on the surface arise internally from the pericycle. of stems 4. The growing apices stems are covered only by bud scales 5. The characteristic appendages of stems are leaves and flowers. The growing apices of roots are covered by root caps. The characteristic appendages of roots are root hairs. 6. Vascular cambium produces same True pith absent, but may have pithsecondary xylem & phloem like parenchyma in center. Pith present 2.3.2. The external morphology of the Gymnosperm plant gymnos – naked sperma – seed We have spent a considerable amount of time on the external morphology of the angiosperms and we wish now to look at the gymnosperms. Gymnosperms are seed plant whose seeds are born exposed and therefore not contained in a fruit as in the case of the angiosperms. The gymnosperm tree, such as the pine tree, is the sporophyte since it bears spores. 2.3.2.1 The classification of Gymnosperms The gymnosperms fall into three orders, as outlined below: 1. Order Cycadales (Cycads or Sago palms) Examples Cycas revoluta and Encepharlatos. These two examples are native to Australia and South Africa. They have been introduced to Kenya in the course of time. Slow growing plants of the tropics. Morphological characteristics of Cycades. (i) They have unbranched stems (ii) The stems have large pith (iii) They have pinnate leaves. (iv) They have very huge female cones, particularly in the case Encepharlatos. Slow-growing plants of the tropics and sub-tropics. Dioecious with massive male and female strobili born on different plants. IMAGE Fig. 2.6 A diagram of encepharlatos plant bearing a huge female cone. 2. Order Ginkgoales. This consists of only one living genus i.e. Ginkgo and one species bibola, making Ginkgo bibola. It is sometimes referred to as the maiden hair tree. Ginkgo bibola tree grows to a sizeable tree of up to 80ft. in height. The stem is woody. The leaves are fan-like in shape. A female plant has several strobili borne at the summit of short stalks, Fig. 2.7 (a) A male tree bears several male strobili, Fig. 2.7 (b) Thus Ginkgo bibola tree is dioecious (male and female reproductive organs borne on separate trees). IMAGE Fig. 2.7 (a) Showing female strobili on a leafy shoot of Ginkgo bibola IMAGE Fig. 2.7 (b) Showing male strobili on a leafy shoot of Ginkgo bibola 3. Order Coniferales This is the most economically important order, with large trees such as: The pine tree (Pinus patula) The cedar tree (Cedrus sp.) The cypress tree (Cupressus sp.) The podo tree (Podocarpus sp.) The coniferous trees are an important source of: - Timber for building construction - Timber for furniture making - As well as for firewood Characteristics of coniferales 1. They have needle – shaped leaves 2. The leaves are evergreen 3. They are either trees or shrubs. There are no herbs. 4. They are monoecious, i.e. male and female cones are borne on the same plant (although some are dioecious with male and female strobili on different plants. 5. The leaves are simple and need-like. 4.gnetales 2.3.2.2 Characteristics of Gymnosperms. 1. They have seed borne on ovuliferous scales and exposed (there is no fruit wall). 2. They have tru roots stems and leaves. 3. The sporophyte plant is a big tree or shrub there are no herbs. 4. Water is not required as a medium of transport of the male gametes (Spermatozoa) in achieving fertilization. 5. Have tracheid xylem elements instead xylem vessels found in angiosperm. 6. The phloem has sieve tubes but have no companion cells. 7. They have starminate (male) cones and ovulate (female) cones on separate plants, i.e. they are dioecious, while others are monoecious. 2.3.2.3 THE REPRODUCTIVE STRUCTURES Male and Female cones of the pine. The pine tree e.g. Pinus patula is one of the economically important coniferous trees. It bears male and female cones on the same plant. The female cones bears woody scales each of which has 2 ovules at the early stages. After fertilization 2 winged seeds are formed. When the cones dry up the scales open up and the 2 winged seeds may be dispersed by the wind, Fig. 2.8 IMAGE Fig. 2.8 showing a small portion of a female cone with opened up female scales bearing 2 winged seeds. 2.4 Questions 1. Give an illustrated account of the external morphology of a young bean seedling. 2. Give a brief description of two types of specialized stems. Illustrates your answer with labeled diagrams. 2.5 Activity 1. Make a study to determine he phyllotaxy of five identified dicotyledonous plants from you local area. Collect leafy shoots of the plants selected, press them and mount them on paper. Identify the plants with a common name and a botanical name (Genus and species), where possible. 2. Make a collection of plants showing or having - a tap root system - a fibrous root system Press the roots then mount on paper and label the specimens 3. Briefly describe two main features that distinguish the gymnosperms from the angiosperms. 2.3.2.4 Main differences between Gymnosperms and Angiosperms 1. The gymnosperms have seeds borne (exposed) on woody ovuliferous scales unlike in the case of the seeds of angiosperms that are found in a fruit, confined or covered by a fruit wall. 2. The xylem of gymnosperms has tracheids while xylems of angiosperms has vessels. Both types of elements are for strengthening the plant. 3. The xylem vessels of angiosperms have open end walls, which facilates the conduction of water up the plant.the tracheids of gymnosperms have no end wall perforations. 4. There are companion cells associated with phoem sieve tubes in angiosperms, but no companion cells are to be found in gymnosperms. 5. Most gymnosperms, particularly among he conifers have needle shaped evergreen leaves unlike the leaves of angiosperms. 2.6 SUMMARY In lesson 2 we have looked at the external morphology of the seed plant; i.e. the angiosperms and the gymnosperms. This has been done with regard to the shoot system and the root system. For shoot system of the angiosperms the following have been highlighted: - Structures on the surface of stem. - Special stems e.g. Tubers, rhizomes and bulbs. - Leaves: simple, compound and complex leaf venation. Leaf arrangement and phyllotax (assignment) For the roots of Angiosperms the following was highlighted. - Types of root systems. Tap root, fibrous and special roots, - The various regions of the root. For the Gymnosperms the following was highlighted - The classification of the gymnosperms, - Characteristics of gymnosperms - (The reproductive structures male and female cones of the pine) - Leaf morphology. The main differences between gymnosperms and angiosperms was also highlighted. 2.7 REFERENCES Esau, K. 1960. Anatomy of seed plants. Esau, K. Plant Anatomy Fahn, A. 1974 Plant Anatomy Foster, A. S. Comparative Morphology of Vascular plants. Fuller, H.J. and O. Tippo. 1949, College Botany. Steeves, T.A. 1989, Patterns in plant development. Weier, T.E., C.R. Stocking and M. G. Barbour 1974. Botany. An introduction to plant Biology. LECTURE THREE 3.0 MERISTEMS AND THEIR ROLE IN THE GROWTH OF PLANTS 3.1 INTRODUCTION In lesson 1 and 2 we mainly described the morphology of plants without going into how plant cells are formed. In unit 3 we wish to introduce to you the study of the meristems in plants. In the early stages of the development of the embryo of a plant all the cells are able to divide. But later the further development cell division becomes restricted to particular parts of the plant. These are the parts with cells that divide and constitute meristematic tissues or simply meristems. 3.2 OBJECTIVES By the end of this unit you will be able to: Define what a meristem is. Describe the main features of a plant cell Describe the different types of meristems Describe the characteristics of metistematic cells. Distinguish the different theories concerning the shoot and root apical organization. (Explain what “totipotency” is) Explain the difference between “differentiations” and “de-differentiation” Describe the different tissues of the stem and root which result from the activities of the meristems. 3.3 DEFINITION meristems are the regions of a plant whose cells keep on dividing i.e regions in which undifferentiated cells divide. 3.4 THE PLANT CELL Before going into the study of the meristematic tissues of plants we would like to describe the main structural features of the plants cell. MISSING PAGES 3.6 THE CLASSIFCATION OF MERISTEMS. The classification of the meristems is based on several criteria. The most important of these are: 3.6.1 Their position or location in the plant body These include: (i) The apical meristems of the shoot and the root. These are also called the terminal meristems which occur at the growing apices of the shoots and the roots. The apical meristem of the shoot is more complex that the apical meristem of the root because it is involved in the initiation of leaf primordial and the lateral branches of the stem. In addition there is no structure equivalent to the root cap found at the shoot apex. The promeristem The distal part of the apical meristem is referred as the promeristem, in which literally all the cells divide. In the peripheral region the cells remain meristematic, and give rise to: The epidermis The cortex The leaf primordial The procambium which in turn give rise to the vascular tissue, as shown in Fig. 3.2. IMAGE Fig. 3.2 Diagram of a longitudinal section of a dicotyledon shoot apex showing the promeristem region. Theories concerning the shoot and the root apical organization. There are three theories which have been proposed to explain the shoot and the root apical organization in plants. These are: a) The shoot Apical Cell Theory This theory was proposed by Nageli in 1858. it postulated the existence of a single cell at shoot apex of pteridophyte called Equisetum (horse tail). The single apical cell is also found in other pteridophytes such as the aquatic fern Marsilea. The apical cell is shaped like an inverted pyramid or a tetrahedron as shown in Fig. 3.3, The apical cell cut off cells along three planes. IMAGE Fig. 3.3 a diagram of a longitudinal section of the shoot apex of a fern showing the apical single cell. b) The Tunica Corpus theory This theory was proposed in 1924. the apices can be divided into two regions of dividing cells, i.e. The tunica on the outside and the corpus on the inside as shown in Fig. 3.4. IMAGE Fig. 3.4 A diagram of longitudinal section of shoot apex of a dicotyledonous plant showing a 2-layered tunica and the corpus. The tunica consists of one or two layers of cells. It has been shown that the cells of the tunica divided anitclinally i.e., approximately at right angles to the dome, on the other cells in the corpus divide periclinally i.e, approximately in a plane parallel to the dome. The tunica corpus theory of the shoot and the root apical organization has proved more acceptable than the histogen theory. c) The histogen theory. This theory was proposed by Hanstein in 1870. the theory postulated that the shoot and root apices consisted of 3-super imposed cell layers (or histogens) which through cell division gave rise to specific regions of the plant body. The 3 layers are shown in Fig. 3.5. IMAGE Fig. 3.5 A diagram of a longitudinal section of a shoot apex showing the 3 layers or histogens i.e. The dermatogen, periblem and plerome. The dermatogen This is the outermost layer of cell whose division gives rise to the epidermis. The periblem This is the region occurring between the dermatogen and the plerome. Cells of the periblem divide to give rise to the cortex of stem and the mesophyll cells of the leaf. Fig. 3.6 shows the three layers of cells in the case of root apex. As in the case of shoot apex, the dermatogen of the root gives rise to the epidermis. The periblem gives rise to the cortex. But the plerome gives rise to vascular system and the pith. IMAGE Fig.3.6 A diagram of a longitudinal section of root apex showing the dermatogen, periblem, plerome and the root cap. (ii) The lateral meristems There are three types of lateral meristems: The fascicular (vascular) cambium This is found in the vascular bundles, occurring between the phloem and the xylem, as in Fig. 3.7 (a) IMAGE (a) Primary tissues IMAGE (b) Portion of stem showing cork cambium and interfasicular cabium. Fig. 3.7 A diagram of a transverse section of a dicotyledonous stem showing fascicular cambium (a) and interfascicular cambium (b) as well as the formation of cork (Phellem) from cork cambium (phellogen). The interfascular cambium This is the cambium formed during the process of secondary thickening of a dicotyledonous stem. Some of the parenchyina cells in the medullary ray region between the vascular bundles undergo de-differentiation and become meristmematic. The interfascicular cambium links up with the fascicular cambium to form a continuous ring of cambium. This cuts off cells on the inside which become xylem, i.e. secondary xylem. This results in an increase in the diameter of the stem (an increase in girth). In big trees annul rings of xylem/wood form. This constitutes what is known as secondary growth. Cork cambium (or phellogen) Cork cambium or phellogen arises fro the outer cortex cells of shrubs and trees through the process of de-differentiation to form a meristematic ring of cells. These cork cambium cells divide periodically. The outer daughter cells differentiate to become cork cells (or phellem) Next round of cell division cuts off cells on the inside which differentiate to form the phelloderm as shown in Fig. 3.7. The cork so formed is impervious to water and is therefore useful to the plant. (iii) The intercalary meristems This type of meristems is found in the form of a band at the region where the leaf sheath joins the stem in a monocotyledonous plant, particularly among the members of family Graminae, Fig. 3.8 This meristem contributes to the elongation of the internoded. IMAGE Fig. 3.8 a diagram showing the intercalry meristem of a Graminae plant such as maize or grass. 3.6.2 The stage of the development of the plant which the meristems appear. On the above basis the meristems fall into two categories. The primary meristems: these are the meristems which develop directly from the embryonic cells, e.g. the fascicular cambium. The secondary Meristems: these are formed when cells of the already differentiated tissues de-differentiate and become capable of cell division. Examples are cork cambium (Phellogen) and the interfascicular cambium already referred before. 3.7 Questions 1. Briefly describe the main structural features of a plant cell. Illustrate you answer with a labeled diagram. 2. What are the characteristics of the meristematic cells? 3.8 Activity Collect from the field shoot apices of 3 different monotyledonous plants and 3 different dicotyledonous plants. Cut longitudinal sections of these apices and examine them with a hand lens and also under the microscope (medium power). Identify and draw the main features. 3.9 Summary In this unit we have looked at: The main structural features of a typical plant cell. Different types of meristems and how they function. The theories concerning plant shoot apical organization. Totipotency, differentiation and de-differentiation and their significance. 3.10 References Esau, K. 1960. Anatomy of seed plants (1962 edition) Esau, K. Plant Anatomy Fahn, A. 1974 Plant Anatomy Foster, A. S. Comparative Morphology of Vascular plants. Fuller, H.J. and O. Tippo. 1949, College Botany. Steeves, T.A. 1989, Patterns in plant development. Weier, T.E., C.R. Stocking and M. G. Barbour 1974. Botany. An introduction to plant Biology. LECTURE FOUR THE MAIN TYPES OF CELLS AND TISSUES FOUND IN PLANTS 4.1 Introduction In the previous lesson we looked at meristems and the way they divide to give new cells. In this lesson we look at the different types of cells and the tissues they form. This is useful because as one studies the anatomy of plant organs such as the stems, roots, leaves and flowers, one constantly comes across one or the other of these cell types and tissues. We shall also find out that these tissues carry out specific functions. 4.2 Objectives By the end of this unit you should be able to: Describe the characteristics of the palisade, collenchyma, sclerenchyma (sclereids and fibres), epidermis, cork, sieve tubes and companion cells. Explain the tissues that these cells and elements give rise to and their main functions. 4.3 The main types of plant cells. The following are the main types of cells in plants. 4.3.1 Parenchyma cells. The parenchyma cell is structurally the simplest cell in a plant Fig 4.1 (a). the parenchyma cells are characteristically isodiametric, with thin cell walls and they are living cell with active protoplasts. The cell walls are composed of cellulose. Parenchyma cells occur in both the simple and the complex tissues of plants e.g. in the stem pith (simple tissue) and xylem (complex tissue). The parenchyma cells play the role of water storage, photosynthesis as in the case of leaf mesophyll and the storage of food materials such as sugars and proteins. They are capable of differentiating and becoming meristematic whereby they divide repeatedly resulting to wound healing. IMAGE Fig. 4.1 (a) T/S parenchyma IMAGE Fig. 4.1 (b) TS Collenchyma 4.3.2 Collenchyma cells. Collenchyma cells are living cells which are characterized by uneven thickening of the cell walls. The cells tend to be more thickened at the corners. Collenchyma cells occur in the stems cortex of herbaceous and are positioned just below the epidermis; Fig. 4.1 (b) Collenchyma cells are absent from the root of monocotyledons except in some cases. The collenchyma cells provide support, particularly to herbaceous plants. The uneven thickening of the cell walls is due to uneven deposition of cellulose, hemicellulose and pectic substances. The mature collenchyma form a strong flexible tissue. 4.3.3 Sclerenchyma cells There are two types of sclerenchyma cells: The sclereids. The sclereids are characterized by having thickened lignified cell walls which may have pits. At maturity the sclereids lose their protoplast and therefore die off. The sclereids are dimensionally isodiametric,slightly elongated or irregularly shaped Fig. 4.2. The main function of sclereids is to provide support. IMAGE Fig. 4.2 Sclereid The other type of the sclerenchyma cells are the fibres. The fibres are also thick walled like the sclereids, but differ from the sclereids in that they are slender and much elongated, and posses pits Fig. 4.3 (a,b). at maturity the fibres just like the sclereids are dead wood fibres. They have heavier cell wall thickening with greater lignification. IMAGE IMAGE Fig. 4.3 (b) L.S. of a wood fiber 4.3.4. Epidemal cells. The epidermis is normally made up of a single layer of cells and it is the outermost tissue in the plant at the primary stage of growth. The outer wall of the epidermal cells is much thickened by the deposition of cutin to form a cuticle Fig. 4.4. this chemical substance, cutin is a waxy water proof substance secreted by the protoplasm of the epidermal cells and serves to reduce cuticular transpiration. The epidermal cells, with the exception of the guard cell have no chloroplasts. The other role of the epidermal cells is to provide protection against mechanical injury, excess heat or cold and attack by parasitic fungi and bacteria. Apart from the guard cells other specialized epidermal cells include those that become trichomes and root hairs. IMAGE Fig. 4.4 TS Leaf. 4.3.5. Xylem tracheids and vessels. The xylem tracheids and vessels are cells which have very much thickened walls for the purpose of providing mechanical strength to the plant as well as water conduction. Te characteristics of the tracheids have already been dealt with in lesson 1, Fig. 1.4 (b) in view of this, in the present lesson we shall only indicate the characteristics of xylem vessels, which are only found in the angiosperms. Xylem vessels. In the evolution of plants it is thought that the tracheid which is found in the pteridophytes and the gymnosperms gave rise to the xylem vessel. This entailed a slight shortening, and widening as well as the perforation of the end walls. So the xylem vessels are characterized by having a greatly thickened cells wall and perforated end walls. Xylem vessels which have undergone secondary thickening show different ways of thickening. In a longitudinal section of the stem of a dicotyledonous plant the following are the forms of secondary thickening that might be observed as in Fig. 4.5. (i) Annular thickening – the first to occur, Fig 4.5 (a) (ii) Spiral or helical thickening which in the order of their formation (ontogenetically) follow annular thickening, Fig. 4.5 (b). These two types of thickening i.e. Annular and spiral thickening, are mainly found in the protoxylem. In the later formed xylem vessels i.e. the metaxylem the vessels are much larger and bands of thickening are much wider giving a ladder like pattern of thickening called the scalariform thickening. Fig. 4.5 (c) Xylem vessels formed later during secondary thickening show a reticulate type of thickening. Fig 4.5 (d). IMAGE IMAGE Xylem vessels, like the tracheids have pits in their thickened cell walls. There are two types of pits, simple pits and bordered pits, fig. 4.6 c, d. a simple pit pair has primary wall and secondary wall, and a pit aperture. The area of the pits is uniform throughout its depth Fig. 4.6 (a). in a bordered pit the secondary wall rises around the pit. The membrane is thicker at the center than at the ends, fig. 4.6 (b). The area of the pit is unequal, being broader towards the original wall and narrower towards the original cavity of the cell. IMAGE IMAGE 4.3.6 Phloem sieve tubes and companion cells. Phloem conducts food from leaves to all parts of the plant. It is made up of sieve cells in the lower vascular plants and sieve tubes and companion cells in angiosperms Fig. 4.7. Sieve tubes are made up of sieve elements connected at the ends with perforated characteristic areas called sieve plates. The pores are surrounded by a colourless substance called callus. The sieve cells of lower vascular plants can quickly accumulate callus to block pores. During an active season like spring in temperature climates, the callus dissolves. Ferns and conifers have sieve areas scattered all over cell wall. Sieve tube cell walls are thin and are made up of cellulose. Sieve tubes are associated with parenchymatous cells called companion cells. Companion cells have a nucleus and cytoplasm and each is connected to the sieve tube through a pore. IMAGE Fig. 4.7 LS of a sieve tube and companion cells. 4.3.7 Cork cells In lesson three we came across the type of meristem called cork cambium or phellogen which we saw produces cork cells on the outerside. The following are the characteristics of cork cells: Their cell walls are greatly thickened with cellulose, and hemicelluloses. These cell walls are also heavily suberized through the deposition of a substance known as suberin. The cells walls are also lignified by the deposition of a substance called lignin. Cork cells, particularly as a result of being heavily suberized are impervious to water, and therefore check excessive water loss. Cork forms the outer bark of stems and roots of woody plants. These are plants which have undergone secondary thickening. 4.4 Types of plant Tissues. A tissue is a group of cells with common origin, structure and function. Permanent plant tissues may be classified into two types as follows: Simple Tissues Simple tissues comprise of one type of cells e.g. pith which consists of parenchyma cells only. Thus simple tissue is homogeneous in its composition. Complex tissues Complex tissues are made up of several different types of cells e.g., xylem. In other words a complex tissue is heterogenous in its composition. We shall give more details about each one of the above two types of tissues: 4.4.1 Types of simple tissues There are several types of simple tissues, as follows: The epidermis This is made up of the epidermal cells, and as noted earlier this is a protective tissue The parenchyma tissue This is made of parenchyma cells and is characteristically found occurring in the cortex, pith and medullary rays of the dicotyledon stem. The parenchyma tissue also occurs in the cortex of the stems of the monocotyledonous plants. Parenchyma is also found in the cortex of the root. The collenchyma tissue. This is found in many dicotyledonous stems, just below the epidermis, forming part of the cortex. It provides the plant with mechanical strength. The sclerenchyma tissue. This tissue is made up of two types of elements, i.e. sclereids and fibres. Sclereids. Sclereids has been shown to occur in the pith of gymnosperms such as Podocarpus. Sclereids or sclerotic cells also occur among leaf mesophyll, and in some fruits, and seed coats of leguminous seeds. Sclereids cause the hardening of the seed coats. Fibres. Fibres have been shown to completely enclose each vascular bundle resulting in a bundle strand. In the dicotyledonous plant fibres are particularly found in the vascular tissue. In the flax plant (Linum usitatissimum) there is a preponderous presence of phloem fibres bordering on the phloem (between the phloem and the cortex). Fibres are economically important in the production of ropes and cords. Another plant that produces phloem fibres is jute (Corchorus capsularis). 4.4.2 Types of complex Tissues Complex tissues are made up of more than one type of cells which work as a unit. They are two kinds: xylem and phloem, both of which are conducting and strengthening tissues of plant. We shall now give some details of the composition, structure, location and functions of these two complex tissues. Xylem The xylem is a major component of the vascular bundle of the stem of both dicotyledonous and monocotyledonous plants. In a collateral vascular bundle the xylem occurs towards the pith separated from the phloem by the vascular cambium. Fig. 4.8 (a). Some plants have what are known as the bicollateral vascular bundles. An example of such plants is the herbaceous of cucumber, A bicacollateral vascular bundle Fig. 4.8 (b) has two sets of phloem, one on the outside towards the cortex and the other on the inside towards the pith. IMAGE Phloem Phloem is composed of sieve tubes, companion cells, phloem parenchyma and phloem fibres. The primary function of this complex tissue is conducting of food to growing regions and storage organs. Phloem cells are living as opposed to the xylem tracheids and vessels which are dead. As explained earlier in this lesson, the composition of the phloem elements is variable between the lower vascular plants and angiosperms. 4.5 Question What structural features and call specialization are associated with basic functions of the complex tissues in angiosperms? 4.6 Activity Apart from water and food conducting tissues, secretory tissues are also important in plants. Read and write notes on the various types of secretory structures, their functions in plant and any economic importance to man. 4.7 Summary There are two types of tissues in plants, simple and complex. Simple tissues include: Parenchyma cells. These are thin walled cells which perform various functions, e.g. food storage and photosynthesis. They are capable of differentiating (they can change activities). Collenchyma cells. These are thick walled living cells thickened at the corners with cellulose and pectin substances, they give mechanical support to the plant. Sclerenchyma cells. These are lignified cells which provide mechanical support. Complex tissues include: Xylem: these include tracheids and vessels composed of dead cells and are used for transport of water. Phloem consists of living cells, sieve cells, sieve tubes and companion cells for conduction of food from leaves to all parts of the plant. 4.8 References Esau, K. 1977. Anatomy of seed plants 2nd ed. New York. Wiley. Fahn, A. 1982 Plant Anatomy. 3rd ed. New York. Pergamon press. Rost, T. L. et al, 1984. A brief introduction to plant Biology 2nd ed. New York. Wiley. Dutta, A.C. 1979. Botany for degree students 5th ed. Calcutta. Oxford University press.