200 



MORPHOLOGY OF GYMNOSPERMS 



but two weeks later they are 0.25 mm. long, the nucellus being 

 homogeneous and not wholly inclosed by the integument (fig. 228). 

 Not all the buds of the dwarf shoots are fertile, and the strobili in 

 those that are range in number from one to six. In such a bud the 

 sequence of structures from outside inward is as follows : brown bud 

 scales, green scale leaves, foliage leaves, strobili, foliage leaves. About 



Figs. 228-230. — Ginkgo biloha: early stages in the development of the ovule; 

 fig. 228, nucellus in April, not yet covered by integument; no archesporium can be 

 distinguished at this stage; X15; fig. 22Q, condition May i; X12; fig. 230, May i, 

 megaspore mother cell surrounded by "spongy tissue"; X 325. — After Miss 

 Carothers (46). 



the first of May the ovules are o. 75 mm. long; the prominent nucellar 

 beak has developed, in some cases protruding from the micropyle; 

 a large pollen chamber has been formed, w^hich often contains pollen 

 grains developing tubes; and the sporogenous tissue is in all stages 

 from immature mother cells to complete tetrads. According to 

 HiRASE (22), the pollen chamber results from the exclusive growth 

 of the external tissue of the beak, the inner tissue thus becoming rup- 

 tured and disorganized. 



Usually one mother cell functions, two functioning mother cells 



