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Dicot stem
Chemistry (Chem345)
University of Sargodha
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Anatomy of a Typical Young Dicot Stem
Example: Helianthus annus (Sunflower)
A transverse section taken through the young stem of Sun-flower reveals the following details.
Sub Topics
1. Epidermis
2. Hypodermis
3. Cortex
4. Endodermis
5. Stele
6. Pericycle
7. Medullary Rays
8. Pith
9. Vascular bundles
10. Diagnostic Features of a Young Dicot Stem
Epidermis
####### Epidermis is the outermost covering of the stem. It is represented by a single layer of compactly
####### arranged, barrel-shaped parenchyma cells. Intercellular spaces are absent. The cells are slightly thick
####### walled. Epidermis shows the presence of numerous multicellular projections called trichomes. Externally,
####### a thin transparent waxy covering called cuticle, which prevents excessive evaporation of water, surrounds
####### the epidermis. The epidermis also contains numerous minute opening called stomata, which are mainly
####### involved in transpiration.
####### T. of a Dicot Stem (Sunflower)
####### Cross-Section of a Dicot Stem
Pith
####### Pith is the innermost part of the stem formed by a group of loosely arranged parenchyma cells.
####### Intercellular spaces are prominent. The pith is also meant for storage of food.
Vascular bundles
####### They are eight in number, arranged in form of a broken ring. The vascular bundles are conjoint,
####### collateral and open. Xylem is on the inner surface and phloem on the outer surface. Xylem is described
####### as endarch.
Diagnostic Features of a Young Dicot Stem
####### Following are some of the diagnostic features of a young dicot stem
####### * Presence of cuticle and trichomes.
####### * Presence of stomata.
####### * Presence of a hypodermis made up of collenchyma.
####### * Presence of a wavy endodermis containing numerous starch grains.
####### * Presence of a bundle cap above each vascular bundle, formed by sclerenchyma.
####### * Presence of eight vascular bundles, arranged in the form of a broken ring.
####### * Presence of conjoint, collateral and open vascular bundles with an endarch xylem.
Stamen
.
Stamens of a Hippeastrum with white filaments and prominent anthers carrying pollen
The stamen (plural stamina or stamens, from Latin stamen meaning "thread of the
warp") is the pollen-producing reproductive organ of a flower. Stamens typically consist of a
stalk called the filament (from Latin filum, meaning "thread"), and an anther (from Ancient
Greek anthera, feminine of antheros "flowery," from anthos "flower"), which contains
microsporangia. Anthers are most commonly two-lobed and are attached to the filament either at
the base or in the middle portion. The sterile tissue between the lobes is called the connective.
A typical anther contains four microsporangia. The microsporangia form sacs or pockets
(locules) in the anther. The two separate locules on each side of an anther may fuse into a single
locule. Each microsporangium is lined with a nutritive tissue layer called the tapetum and
initially contains diploid pollen mother cells. These undergo meiosis to form haploid spores. The
spores may remain attached to each other in a tetrad or separate after meiosis. Each microspore
then divides mitotically to form an immature microgametophyte called a pollen grain.
The stamens in a flower are collectively called the androecium (from Greek andros
oikia: man's house). The androecium forms a whorl surrounding the gynoecium (carpels) and
inside the perianth (the petals and sepals together) if there is one. (The one exception is a few
members of the family Triuridaceae, particularly Lacandonia schismatica, in which the
gynoecium surrounds the androecium.)
Contents
1 Variation in stamen morphology
2 Descriptive terms
3 Sexual reproduction in plants
4 Gallery
5 References
Variation in stamen morphology
Stamens can be free or fused to one another in many different ways, including fusion of
some but not all stamens. The filaments may be fused and the anthers free, or the filaments free
and the anthers fused. Rather than including two locules, one of the locules may fail to develop,
or alternatively the two locules may merge late in development to give a single locule. Extreme
cases of stamen fusion occur in some species of Cyclanthera (of family Cucurbitaceae) and in
section Cyclanthera of genus Phyllanthus (family Euphorbiaceae) where the stamens form a ring
around the gynoecium, with a single locule.
Descriptive terms
Scanning electron microscope image of Pentas lanceolata anthers, with pollen grains on surface
A column formed from the fusion of multiple filaments is known as an androphore.
The anther can be attached to the filament in two ways:[3]
basifixed: attached at its base to the filament
dorsifixed: attached at its center to the filament, usually versatile (able to move)
Stamens can be connate (fused or joined in the same whorl):
monadelphous: fused into a single, compound structure
diadelphous: joined partially into two androecial structures
pentadelphous: joined partially into five androecial structures
A flower having only functional stamens and lacking functional carpels is called a
staminate flower, or (inaccurately) male. A plant with only functional carpels is called pistillate,
or (inaccurately) female.
An abortive or rudimentary stamen is called a staminodium or staminode, such as in
Scrophularia nodosa.
The carpels and stamens of orchids are fused into a column. The top part of the column is formed
by the anther. This is covered by an anther cap.
Pollen Development
Pathways of pollen development
Pollen is produced within the anthers (microsporangia or pollen sacs) of the flower.
During its development from an undifferentiated mound of cells (anther primordium) the anther
forms two general groups of cells. The reproductive or sporogenous cells give rise to the
microspores and are formed from cells located centrally within the developing anther. The non-
reproductive cells form discrete anther tissues layers and include the epidermal, cortical and
tapetal cell layers surrounding the sporogenous cells. The tapetum which is the innermost layer
of the pollen sac plays a dominant role particularly during the microspore stage. For example,
many male sterile mutations affect tapetal cell functions and development is often arrested during
the microspore stage.
Microsporogenesis and microgametogenesis
Two distinct and successive developmental phases, microsporogenesis and
microgametogenesis, lead to the production of the mature microgametophytes.
Dicot stem
Course: Chemistry (Chem345)
University: University of Sargodha
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