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PLANT TISSUES Embryogenesis First asymmetric division of zygote determines a polarity, i.e. the structural axis (or growth axis apexbase) of the plant corm. Seed The seed is a fertilizes ovule produced by sexual reproduction. It allows plant to withstand adverse environmental conditions and to facilitate the diffusion of the species. Seed Dormancy is a resting condition with reduced metabolic rate. This is found in non-germinating seeds and non-growing buds. Dormancy protects the organism against adverse conditions and occurs before their onset. It increases probability of a successful germination. To stop dormancy the following factors are requested: water, O2, suitable temperature and photoperiod, exposure to cold, impairment of seed integument. Germination is under the influence of plant hormones such as gibberellin and abscissic acid. Seed germination Germination stages of Dicotyledones Germination stages of Monocotyledones •Period between germination and seedling indipendence is a crucial phase since the plant is susceptible to attacks by insects and microorganisms and to hydric stress. •Plant growth can be seen as a sort of movement. Plants move through elongation of roots and stems. In this way plant modifies its relation with environment, by curving towards the light or producing roots absorbing soil water. Meristems Seed during germination (SEM) •Most of cell divisions in the plant happen in apical meristems, a group of cells that are capable of dividing indefinitely and whose main function is the production of new growth. They are found at the growing tip of a root or stem. In addition cell divisions happen in lateral meristems (vascular and cork cambium) giving secondary growth. Bud tip in longitudinal section Apical meristems Root tip in longitudinal section What is a plant tissue? •An aggregation of cells with a similar form and function that form the material of which a particular part of a plant is composed. Origin of meristems and primary plant tissues Protoderm Apical meristem Fundamental meristem Procambium Epidermis (epidermal tissues) Parenchyma, collenchyma and sclerenchyma (fundamental tissues) Primary xylem and phloem (conducting tissues) Primary meristems •Meristematic cells continue to undergo cell division throughout the life of the plant. •Thay are isodiametric and have thin primary cell wall and dense cytoplasm; they have not a large vacuole. •They are totipotent, i.e. are capable to give raise to different kinds of adult tissues. Kinds of adult cells that can be originated by a procambium cell. Adult tissues •Adult cells have a central vacuole occupying most of internal volume and pushing all contents of the cell’s cytoplasm against the cellular membrane. •They have differentiated different organelle necessary to specific functions. •They possess a secondary cell wall and show modifications to the wall composition. •Distribution of conducting, fundamental and epidermal tissues in the plant organs Pharmaceutical importance of plant tissues •Numerous plant drugs come from different parts such as leaves, stems, roots, rhizomes, fruits, etc. This material is often reduced to small pieces or powders so that it is difficult to identify by macroscopic analysis. •A good knowledge of plant anatomy and tissues is fundamental to recognize plant drugs and to obtain safety and quality standard needed for their use (see Pharmacopoeia). Fundamental tissues Parenchyma •Parenchyma is a succulent tissue, consisting of more or less isodiametric, thin-walled cells, often with intercellular spaces, that is found in e.g. the softer parts of leaves, the pulp of fruits and the pith of stems. •Main functions are photosynthesis, storage, secretion, respiration, etc. •Parenchyma cells are regarded as the basic cells from which all other cell types have evolved (totipotent). •They form a large part of the bulk of many organs. Collenchyma •Collenchyma is a specialized form of parenchyma, with unevenly thickening in the cell walls (primary). •It provides strenghthening and support for primary structures (i.e. those lacking secondary thickening) such as young shoots (e.g. cortex of stems) and leaves (e.g. leaf veins and petioles). Angular collenchyma Collenchymatic cell at SEM •Main feature of collenchyma is the evenly thickening and without lignin of primary cell wall that remains soft and flexible. Sclerenchyma •It is the fibrous or woody tissue in a plant that provides mechanical support. •The tissue is formed by cells whose walls (secondary) are thickened with cellulose or lignin, and when mature the cells usually contain no living protoplasm. •The cells may occur singly or in groups and may be long and fibrous or stony. •Such cells can be found in the cortex, phloem and xylem. Sclerenchyma •Stone cells (sclereids) are found in seed integument, nut shell, kernel and pulp of some fruits (e.g. of pears). Sclerenchyma •Fibres are elongated cells with tapered ends occurring in groups. Used in textile industry are fibres of hemp, jute and linseed, all derived from stems of Dicotyledones. Conducting tissues •In the vascular plants, xylem is the main tissue conducting water and minerals from roots towards the leaves, phloem of nutrients (photosynthates) from leaves to other parts of the plant. •They aggregate in groups of vascular bundles forming a continuous conductive system throughout the plant along which water and soluble nutrients pass. This vascular system also contributes to the structure support of the plant. conducting bundle fibres Xylem •Xylem consists of various types of cells, vertically stacked. When mature they are dead, with tick, lignified walls. trachea tracheids Xylem •Trachea is an open vessel, large in diameter, short in lenght, of a tube-like series of non-living cells. It presents a perforation plate, the end wall of vessel element, with 1 or more openings (perforations) to allow the passage of water and dissolved substances. •Tracheid is a closed vessel, long, narrow, cylindrical, tapered, dead cell with bands of lignin. It is typical of Gymnosperms and other woody plants. It does not present perforations, but only pits so that the passage of water and salts is slower than through perforation plates. Xylem Angiosperme Dicotiledoni Gimnosperme •Conducting efficiency of xylem cells. Trachea with simple perforation Scalariform perforation External surface of a trachea at SEM annular and spiral trachea double spiral trachea Differentiation of a trachea Phloem •Phloem comprises various types of cells which lie end to end forming a tube (sieve tube) and transport dissolved organic and inorganic materials over long distances within vascular plants. •Phloem can be distinguished from xylem by the general absence of thickened cells (no dead) and by the presence of cells containing areas resembling a sieve (sieve plates). longitudinal section of secondary phloem of yew sieve area sieve tubes longitudinal section of secondary phloem of linden companion cells sieve plate Sieve plate It is a region of pores that perforate the end of a sieve cell. Cytoplasmic strands may pass through the pores, interconnecting adjacent sieve cells and facilitating translocation. A sieve area is occasionally present in the lateral wall of a sieve cell. Sieve plates occur in the phloem of Angiosperms, while Gymnosperm possess sieve areas that are less efficient, due to minor openings, in the transport of organic substances. Sieve cells •Long, slender, tapering cells which form the sieve tube. To allow the passage of water and photosynthates they lack nucleus, vacuole, ribosomes, Golgi body, microtubules, but retain cytoplasm along the wall. Each sieve cell ends in a sieve plate. •Unlike xylem elements, phloem cells are not dead at maturity. sieve plate Companion cells •They support sieve tubes. •They possess complete protoplast with nucleus and organelles. •They are connected with sieve cells by numerous plasmodesma. •They play an important role in the loading of organic substances to be transported in the sieve tubes. sieve tubes and companion cells P protein Sieve tubes and companion cells in trasversal and longitudinal sections Differentiation of a sieve tube Epidermal tissues •They are represented by epidermis, the outermost layer of cells (one cell thick) in plant, where it forms a protective covering to leaves, flowers, fruits, seeds, and also primary stems and roots. •In woody plants epidermis is replaced by periderm. •It is composed of flat, against each other, chloroplast-free cells (with the exception of guard cells of stomata). •Epidermal cells have only primary wall, whose outer layer is impregnated with cutin and waxes, forming cuticle, which reduce water loss. •Root epidermis is called rhizoderm; it is devoid of cuticle. •Epidermis may bear stomata (small openings allowing access for carbon dioxide and egress of water); guard cells (surrounding and controlling the pore size); trichomes or hairs (outgrowth from an epidermal cell, involved in absorption, protection, secretion, sunlight-protection, reducing temperature and transpiration). Epidermis with stomata stoma of corn •Stomata are found most often on the lower surface of leaves but also on young stems, which allow gases to pass in and out of the plant. stoma of corn in trasversal section stoma surrounded by waxes at SEM Pharmacognostic importance of stomata •Stomata may be useful to identify a powdered plant drug. •The main stoma types are also included in the Italian Pharmacopoeia together with the stomatal index: Pharmacognostic importance of stomata 1 2 4 3 anomocitic anisocitic diacitic paracitic Lower epidermis with diacitic stomata and glandular hairs Trasversal section of leaf with branched trichomes and glandular hairs branched trichomes stinging hair of nettle •Periderm is a secondary protective tissue replacing epidermis as coating in older stems and roots. •It is composed of a layer of cork (or phellem, whose wall cells contain suberin), cork cambium (or phellogen) and phelloderm. Classification of plant tissues on functional basis secretori Parenchymatous tissues Epidermal tissues Secretory tissues Secretory tissues External (of epidermal origin, they discharge secrets outside the plant): Epidermal cells (e.g. petals, buds) Glandular hairs (e.g. sage, rosemary, absinth, nettle, Cannabis, etc.) Floral nectars Extrafloral nectars (e.g. acacia) Internal (of parenchymatous origin, they maintain secrets inside the plant): Oil idioblasts (e.g. laurel, camphor) Resin channels (e.g. conifers) Laticiferous vessels (e.g. rubber, euphorbia, poppy) Lysogen or schizogen pockets, gap or spaces (e.g. orange, lemon) Modality of secretion lisogen pockets schizogen pocket Glandular cells may die when discharge secret, or stay live. Secret is discharged by active transport or simple diffusion. Sometimes secret is ejected by inclusion in vesicles or grains. Mechanical tissues Conducting tissues Phloem Xylem Conducting tissues Conducting tissues (Gimnosperme) (Angiosperme) (Gimnosperme) (Angiosperme) (Angiosperme) Conducting tissues closed collateral concentric periphloematic opened collateral opened bicollateral concentric perixylematic Types of vascular bundles Summary Summary Summary