330 



BOTANY 



The Embryo 



Within the egg-cell the nucleus of the sperrnatozoid fuses with that of the egg. 

 The nucleus thus formed gives rise to many free nuclei (Fig. 292, B), and the 

 cell-formation in the young embryo in Cycas is much like that of the gaineto- 

 phyte. The lower part, only, of the egg, forms the embryo, the upper portion 

 remaining as a sac. A very long suspensor is developed, and the embryo 

 develops two thick cotyledons, which remain in the large seed when it germi- 

 nates, the first foliage-leaf arising between them (Fig. 292, E). 



Where fertilization is not effected, the gametophyte has been observed to 

 continue its growth, and protrude from the ovule as a mass of green tissue, a 

 condition unknown elsewhere among the Spermatophytes. 



- B 



FIG. 292. A, Cycas revolnta, upper part of archegonium (x 60) ; 6, ventral canal- 

 cell. B-E, C. circinalis. B, fertilized egg-cell, showing an early stage in the 

 development of the embryo (x 15). C, an older embryo (x 6). D, a still older 

 one, showing suspensor and two cotyledons. E, germinating seed (X J) ; the 

 cotyledons remain in the seed ; r, the large tap-root of the young sporophyte. 

 (A, after IKENO. B-E, after TREUB.) 



Order II. Ginkgoales 



The second order of the Gymnosperms includes but a single plant, 

 the curious "Maidenhair-tree," Ginkgo biloba (Fig. 294), of China, 

 but which is extensively cultivated in Japan, and to some extent in 

 the United States. It is a large tree, which has usually been asso- 

 ciated with the Coniferse, from which it differs in several important 

 respects, especially the character of the gametophyte, which is much 

 more like that of the Cycads. 



The leaves are deciduous, and in their form and dichotomous vena- 

 tion are much like those of such Ferns as Adiantum. The trees 

 are dioecious, like the Cycads, and the structure of the ovule and ripe 

 seed is much the same as in the Cycadaceae. The pollen-sacs are 



