398 



MORPHOLOGY OF GYMNOSPERMS 



merges insensibly into the root cap. There are thus three stages of 

 suspensor-formation : the first elongation in connection with the 

 nucleus which later becomes the nucleus of the embryo cell; the second 

 and chief elongation in connection with the smaller nucleus of the 

 proembryonal cell; and the formation of a secondary, multicellular 



suspensor by the basal cells of the embryo. 

 Although several embryos may begin to 

 develop in a single ovule, only one of them 

 reaches maturity. 



In Ephedra trijurca the occurrence of 

 vivipary is evident, at least in some cases, 

 numerous seedlings being observed to have 

 emerged from the seeds while the strobili 

 were still attached to the stem. In any 

 event, the period of rest for the embryo 

 must be very short, and favorable conditions 

 for growth may eliminate it entirely. The 

 whole period from the first appearance of 

 the ovulate strobili to the "germination" of 

 the seed is approximately six months. 



In Welwitschia the fertilized egg, within 

 the prothallial tube, behaves as does the pro- 

 embryonal cell of Ephedra. As described by 

 Pearson (27), it elongates toward the top 

 of the endosperm, and the first nuclear 

 division is followed by a cleavage wall which 

 cuts off a terminal embryo cell. The 

 general statement has been that in Wel- 

 witschia and Gnetum there is no free nuclear 

 division in the first stages of embryo-forma- 

 tion, but that the first division of the fusion 

 nucleus is accompanied by wall-formation, as in the angiosperms. 

 This statement must be revised, for in W elwitschia the fusion nucleus 

 produces two free nuclei, which are later separated by a wall that holds 

 no relation to the spindle of the fusion nucleus. The suspensor cell of 

 Welwitschia elongates extremely, becoming much coiled and persistent, 

 and the embryo cell produces a pyramidal group of cells (about 



Fig. 454. — Welwitschia 

 mirabilis: a young embryo 

 consisting of a pyramidal 

 group of cells, the basal 

 ones giving rise to embryonal 

 tubes. — After Strasburger 

 (3). 



