THE GYMNOSPERMAE : CYCADALES, ETC. 741 



ovule, though there is not enough evidence to show how they were arranged 

 on the strobilar axis. 



The ovule itself is very like that of Cycas. There is a thick integument, 

 with the same three layers. There is one striking difference however : the 

 integument in spite of its massive development has no vascular system. A 

 concentric bundle from the axis splits into two below the ovule and the 

 branches pass through the hard integument and spread out below the nucellus 

 formiing a mantle of short reticulate tracheids which covers the whole internal 

 surface of the integument, as in some seeds of the Pteridosperms. The free 

 apex of the nucellus breaks down into a pollen chamber and some of the 

 mucilaginous material thus formed oozes out of the micropyle to form a 

 pollination drop. The whole development of the megaspore, prothallus 



Fig. "j^^.— Ginkgo biloba. Transverse section of the 

 ovuliferous shoot showing four bundles. 



and archegonia also closely resembles that of Cycas, except that there is a 

 definite ventral canal cell formed in the archegonium. This cell may later 

 break down and its nucleus re-fuse with the oosphere nucleus, as may also 

 happen in Piuiis. 



The young prothallus passes through a period of free nuclear divisions, 

 usually eight or more, followed by centripetal growth of cells. The cells 

 coming from opposite sides of the megaspore meet but do not join, so that a 

 plane of cleavage is left down the middle of the prothallus. The whole 

 prothallus is elliptical, not circular, in transverse section. The growing 

 prothallus absorbs the spongy tissue of the nucellus around it and also most 

 of the nucellar apex above it. It contains some chlorophyll, which might be 

 regarded as a notably primitive feature, but is probably only a response to 

 the penetration of light through the relatively thin stony layer of the integu- 

 ment, just as chlorophyll will develop in the pith of a translucent stem. 

 There is a very thick megaspore wall and a delicate prothallus wall within 

 it, which are quite distinct from each other. 



