722 A TEXTBOOK OF THEORETICAL BOTANY 



This agrees with the relatively primitive structure of Cycas, a smaller output 

 being characteristic of the more specialized forms, such as Zamia. 



Each sporangium develops around a single archesporial cell, which 

 arises in the hypodermis of the sporophyll. The sporangial wall is massive 

 and several layers thick. Its epidermis later becomes highly thickened and 

 two rows of elongated cells mark the line of dehiscence. There is a scanty 

 tapetum which shrivels up at maturity along with all the inner layers of 

 the wall called collectively the endothecium. 



The mature sporangium is oval and attached by a short stalk at one end. 

 The dehiscence is longitudinal, as in Marattiaceae, and as in the latter there 

 is no organized annulus. The outer thickened layer of the wall, the 

 exothecium, indeed, functions as a generalized annulus, w'hich shrinks 

 with the loss of water from its cells and causes the splitting open of the wall 

 along the abaxial side, where the cells are smaller and thinner, so that the 

 open sporangium becomes boat-shaped and the spores easily fall out. The 

 cells of the exothecium on the adaxial surface are thinner walled than those 

 below and act as a contractile " hinge " in the opening of the wall. Function- 

 less stomata are present near the base of the sporangium, recalling the foliar 

 nature of the sporophyll from which it arises, the epidermis of which is con- 

 tinuous with the exothecium. Sporangial stomata have, however, been lost 

 in all other Gymnosperms and in the Angiosperms. 



Each spore mother cell at meiosis forms a cell with four lobes separated 

 by thickened partitions, and inside each lobe of this structure a microspore 

 develops. 



The microspore has two spore coats with thickenings at opposite poles. 

 The extine is thickest at one end and tapers gradually towards the other 

 end of the spore. The intine is thickest at the sides. Germination begins 

 before the liberation of the spore. A lateral prothallus cell is cut off, which 

 persists. The remainder of the cell divides again forming an antheridial cell 

 attached to the prothallus cell and a tube cell with a very large nucleus. At 

 this stage the spore is shed. The spores are light and dry, and field observa- 

 tions support the view that they are wind borne. 



The Female Strobilus 



The megasporophyll of Cycas revoluta is one of Nature's curiosities. 

 It preserves in a living Seed Plant the image of the seed-bearing fronds of the 

 Palaeozoic Pteridosperms. There is no properly organized cone, only a 

 crown of sporophylls formed seriatim with the leaves, in acropetal succession 

 (Fig. 727), leaving the apical meristem unaffected, to grow on and form 

 future leaves and sporophylls. Young plants do not produce sporophylls, 

 and the growth of Cycas is so slow that it may be many years before the 

 plant reaches maturity, but after it arrives at that stage sporophylls are 

 formed each year between successive crowns of foliage leaves. They are far 

 more numerous than the leaves, and their persistent bases contribute more 

 to the stem armour. 



