200 



GYMNOSPERMS 



determined. When spurs do not bear reproductive organs, the 

 leaves are often lobed. 



The young ovules break through the bud scales and pollination 

 occurs late in April or early in May, while the leaves are still im- 

 mature (fig. 215). 



In northern Ohio the bud of the spur begins to swell about April 

 I. By dissecting away the outer brown bud scales and the inner 



Fig. 215. — Ginkgo biloba: spur shoots with ovules and young leaves, shortly after 

 pollination. In the photo at the right, the leaves have been cut away. The collar is con- 

 spicuous at this stage; X1.5. — After Coulter. '53 



greenish scales, one can see the pale, cream-colored ovules and the 

 very young greenish foliage leaves. At this time the single integu- 

 ment has appeared, but has not begun to cover the nucellus. By 

 May I, sections show the megaspore mother-cell surrounded by the 

 spongy tissue characteristic of gymnosperms (fig. 216). On the same 

 tree, at the same time, some of the mother-cells have divided, giving 

 rise to linear tetrads. Sometimes the mother-cell gives rise to a row 

 of three cells, one of the first two cells produced by the mother-cell 

 having failed to divide. In such a case, only two mcgaspores are 

 produced ; and in any case only one megaspore functions. Occasion- 

 ally, there are two megaspore mother-cells, one above the other, 



