GINKGOALES 



49 





are not sufficiently known. We have been able to secure almost 

 a complete series showing the general outlines of the develop- 

 ment (Figs. 34-36), which merely confirms the facts already 

 published. Germination of the oospore begins, as is usual, 

 among Gymnosperms, with repeated nuclear division (Fig. 

 34, D). These nuclei, however, instead of organizing a parietal 

 tissue as in the Cycads, or a basal group as in the Conifers, 

 proceed to fill the whole cavity of the enlarging oospore with 

 free nuclei, which is followed by the organization of a com- 

 pact tissue (Fig. 34, E). In a certain sense this structure would 

 seem to represent the proembryo of Cy- 

 cads, but it really represents the whole 

 product of the oospore, in which proem- 

 bryo, suspensor, and embryo proper are not 

 differentiated. The complete filling of the 

 spore with tissue, and the lack of early 

 differentiation into the great embryonic 

 regions, would suggest a habit more primi- 

 tive than in either Cycads or Conifers. At 

 the same time, it may be merely a derived 

 character. In any event, the tissue near 

 the base of the spore, which in the other 

 groups develops both suspensor and embryo, 

 shows far greater vigor than the remain- 

 ing tissue. In the organization of the em- 

 bryo the whole mass of tissue is involved, and in the absence 

 of a suspensor the embryo invades the endosperm by direct 

 growth. 



The two cotyledons are differentiated early in October, and 

 are quite unequal in length. The larger one is two-lobed, while 

 the shorter one is cleft halfway down, thus early indicating the 

 bilobed character of the leaf. The two cotyledons are also 

 united at the apex, but the epidermal layers of the two are 

 distinct where they are in contact. The plumule is very con- 

 spicuous between the elongated cotyledons, three or more leaves 

 appearing just behind the stem apex (Fig. 35). 



As in the Cycads, the seed becomes plumlike, a testa with 

 fleshy outer and stony inner layers being organized (Fig. 36). 

 Even without pollination the seed attains its usual size and the 

 two layers of the testa are developed. 



FIG. Zb. Ginkgo biloba, 

 a seed with embryo in 

 stage shown in Fig. 

 35, C: e, embryo; g, 

 endosperm ; k and /, 

 hard and fleshy layers 

 of the testa ; natural 



