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Microsporangium

1) The document discusses microsporogenesis, the development of microspores from microsporangia. Microsporangia produce microspore mother cells that undergo meiosis to form four microspores. 2) It describes the structure of a mature anther wall, which has four layers - epidermis, endothecium, middle layer, and tapetum. The tapetum is important as it provides nutrients to developing sporogenous tissues. 3) Microsporogenesis involves mitotic and meiotic nuclear divisions of microspore mother cells within the anther to ultimately produce four microspores per cell, which then develop into pollen grains. Some key stages mentioned are

Education◦
Presented by:- Gosai Rajeshwariben v. M.sc sem-2 (BOTANY) Paper –CBO 404 Roll no : 6 Department of life sciences, HNGU Patan.
● Introduction. ●Anther Wall ● Sporogenous tissues ● microsporogenesis
→ In considering the course of events leadi -ng to the origin of the embryo,we must first deal with the development of the micro and megasporangia. → It is the microsporangium which produces the microspores and eventually the male gametophyte.
→ Microsporangium gives rise to male game -tophytes. → Microsporangia are notable in spikemosse -s and minority of ferns. → In gymnosperms and angiosperms , the microsporangia produce the microsporo -cyte, also known as the microspore mother cell, which then creates four microspores divide through mitosis to create pollen grains. → The term is not used for bryophytes.
→ A mature anther wall consists of four layer (a) Epidermis (b)Endothecium (c) Middle layer (d)Tapetum (a) Epidermis -outer layer -it undergoes anticlinal divisions only to keep pace with the rapidaly enlarging internal tissues. -protection.
Fig: stages in the development of anther (b) Endothecium: -just below the epidermis. - usually single layer , multilayer (Nicotiana tabacum.)
- The cells of the endothecium become radially elongated and form their inner tangential wall fibres run upwards ending near the outer Wall of each cell. -High content of cellulose material. -The endothecial layer is responsible for the dehiscence of the anther due to differential hygroscopic expansion of the outer and the inner tangential wall. (c) Middle layer : -Next to the endothecium. -1-3 layers of cells constituting middle layer or there may be several as in Lilium. -As a rule, all of them become flattened and crushed at the time of meiotic division in the microspore
mother cell, however , there are a few exceptions where outermost layer persists for a long time. -The cells are generally rich in reserve foods such as starch , thus suggesting its nutritive role in pollen development. (d) Tapetum: -The innermost layer. -Its considerable physiological importance because all the food materials entering into the sporogenous tissues pass through it. -composed single layer , which shows dese cytoplasm and distinct nucleus. →NUCLEAR DIVISION OF THE TAPETUM 1. Endomitosis 2. Retitution nuclei 3.Polytenu
→ TYPES OF TAPETUM 1. Amoeboidal tapetum. 2.Secretary or glandular.
→ The primary sporogenous layer cells give rise to the microspore mother cells or the pollen mother cells. → The sporogenous cells in normal way divide several times mitotically before functioning as pollen mother cells. → In certain cases the primary sporogenous cells may either show only few division or no division at all , thus giving rise to a very small group of sporogenous cells or the primary sporogenous cells themselves function directly as the microspore mother cells.
In the beginning of their formation the microspore mother cells or the pollen mother cells remain closely packed but as the anther enlarges in size, the pollen sac also increases in size , the microspore mother cells also enlarge in size, become spherical in shape and get loosely arranged. → some of sporogenous cells are nonfunctional and serve as the food material for the functional m.m cells . Fig: sporogenous tisses
→The microspore mother cells which are at first polygonal and closely packed gradually become rounded and loosely arranged in the rapidly enlarging microsporangium. → Although all of the mother cells in an anther are capebale of giving rise to pollen grains but some of them may degeneratre and serve as food material for the remain -ing cells which give rise to pollen grains. → Each functional spore mother cell produces four micro -spores or pollen grains.
Fig: Microsporogenesis
→ Cytomixis: - while studying oenothera gigas and O.blennis ,gates for the first time observed a frequent migrastion of chromatic material from one microspore mother cell into another, and called it cytomixis. - This phenomenon has also been reported in several other plants. - In coreopsis tripteris, cytomixis occurs at the end of the meiotic divisions but the multi nuleate cells formed in this way again break up into smaller units of one or two nuclei.
→ Cytokinesis: Fig: Formation of megaspores from a megaspore mother cell
- This division of the microspore mother cells may be of the successive and simultaneous type. -In the successive type, a cell plate is developed immedi -ately after the first meiotic division and another in each of the two daughter cells after he second meiotic division. -However, in the simultaneous type, no wall is formed after the first division and the mother cell becomes separated into four parts when the meiotic divisions are over.
→ A TEXTBOOK OF BOTANY ANGIOSPERMS -B.P PANDEY → WWW. GOOGLE .COM →
Microsporangium