Download sieve tube

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