742 



A TEXTBOOK OF THEORETICAL BOTANY 



Archegonia begin to develop even before the prothallus tissue is com- 

 plete. Only two are formed, side by side, in the plane of the longest diameter 

 of the prothallus. They are short and broad, quite different in shape from 

 those of Conifers and Cycads (Fig. 745). 



Instead of the archegonial chamber of Cycas a ring-shaped trough is 

 formed, surrounding a central beak which supports the remains of the nucellus. 

 It is called the tent pole, and a similar structure is prominent in some 

 Pteridosperm seeds. 



The ovuliferous axes begin to appear in April, at the same time as the 



Neck cells 



ii»i Oosphere nucleus 







Fig. 745. — Ginkgo biloba. Longitudinal section through 

 the apex of the female prothallus showing an 

 archegonium. 



male catkins. At first they grow very rapidly and pollination takes place 

 about the beginning of May, when only the megaspore mother cell has been 

 formed. Fertilization is delayed until late in September, i.e., four months 

 from pollination, as in Cycas. 



The pollen grains in the pollen chamber soon begin to germinate. The 

 extine does not cover the whole cell, and there is a thin portion covered only 

 by the intine, which now protrudes and forms an haustorial tube, burrowing 

 into the nucellus, as in Cycas. At the pollen grain end this tube is enlarged 

 and almost bladder-like and it pushes the grain down through the floor of 

 the pollen chamber, which disintegrates into the space above the archegonia. 



Meanwhile the antheridial cell has divided into a stalk cell and a body 

 cell, while the latter divides again, vertically, to form two cells, in each of 

 which an antherozoid develops (Fig. 746). These are ver>' like those of 

 Cycas, but not quite so large (about 8o/x as against 250/x), and they are more 

 elongate, but they have the same spiral blepharoplast with hundreds of cilia. 

 They are apparently liberated as in Cycas, but details are uncertain. 



