Chapter 8 



ARCHESPORIUM 



Early in the development of the ovule primordium, just before, or 

 accompanying, the appearance of the integument primordia, one to 

 several hypodermal cells at the apex of the primordium are set off from 

 surrounding cells by increase in size, larger nuclei, and denser cytoplasm. 

 These cells form the archesporium — in crassinucellate ovules, a small 

 cluster or plate of cells; in tenuinucellate ovules, usually a single 

 cell, terminating an axial row of cells (Fig. 102B). In crassinucellate 

 ovules, the archesporial cells divide, chiefly periclinally, and form, ex- 

 ternally, primary parietal cells and, internally, sporogenous cells (Fig. 

 111). In tenuinucellate ovules, the archesporial cells commonly be- 

 come sporogenous cells without division and later mature as megaspore 

 mother cells (Fig. 102C), the cells that form the tetrads of megaspores. 

 The terms archesporium, sporogenous tissue, and primary sporogenous 

 cells have been loosely used; they apply to early stages in the elabora- 

 tion of the meristematic tissue from which fertile cells are differentiated, 

 and lines of separation cannot be drawn. In the ovules of many taxa 

 that have little or no nucellar tissue and one or two archesporial cells, 

 the archesporial cells become the spore mother cells. 



Parietal tissue, developed from the primary parietal cells, varies from 

 few to many cells; if there is only one cell, there may be no prolifera- 

 tion, as in many specialized ovules. Divisions may be periclinal only, or 

 both periclinal and anticlinal, and massive nucellar caps may be built 

 up. In these caps, dominant periclinal cell divisions may differentiate a 

 calotte. Periclinal divisions in the epidermis are infrequent and add 

 little to the bulk of large nucelli. Where the ovule primordium is small, 

 only one archesporial cell is usually formed, and this is terminal on a 

 median row of cells sheathed by the epidermis; parietal tissue is absent 

 or consists of very few cells. 



The sporogenous cells are commonly few, but range in number from 

 one or two to ten or more. Casuarina and Cah/canfhus have eight to ten 

 in a central, more or less columnar, cluster, which is not clearly limited 

 (Fig. 111). The large number, together with cells apparently transi- 

 tional to parietal cells, suggests derivation from an ancestral multi- 

 cellular sporogenous cluster. In many primitive and some advanced 

 families, the number is two to several; in most advanced families and 



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