SPERM ATOPHYTES 



209 



FIG. 468. Ovulate strobili of Ginkgo, borne in clusters 

 on dwarf branches, and each bearing two ovules. 



Ovulate. The ovulate strobili are also borne on the dwarf branches 



and are very much reduced (fig. 468). A strobilus consists of a long 



stalk, near the tip of 



which usually two ovules 



are borne, 'only one of 



which usually matures 



a seed (fig. 469). At 



the base of each ovule 



there is a little cupule or 



collar, which is the rudi- 

 ment of the sporophyll 



(fig. 470). Sometimes 



there are three or four 



ovules on a strobilus, 



and sometimes the collar 



becomes leaflike; so it 



is evident that the strobilus is remarkably reduced, usually producing 



only two ovules upon very rudimentary sporophylls. 

 Ovules. The ovules resemble in general structure those of Cor- 



daitales and the cycad line al- 

 ready described (fig. 470), with 

 the three-layered integument 

 (outer fleshy, stony, and inner 

 fleshy layers), the nucellar beak, 

 and the pollen chamber ; but the 

 set of vascular strands, which in 

 the groups referred to traverse 

 the outer fleshy layer, are not 

 present in Ginkgo, only the inner 

 set appearing in the inner fleshy 

 layer. 



The megaspore mother cell is 

 first observed deep within the 

 nucellus, and it becomes invested 

 by a distinct zone of glandular 

 tissue. This glandular zone is 

 of Ginkgo digestive in function, invading 



FIG. 469. Ovulate strobili 



bearing developing seeds; the strobili to the destroying the surrounding 



right have developed single seeds; those to 



the left have developed both seeds. 

 C. B. & C. BOTANY 14 



tissue of the nucellus. 



