360 



PLANT MORPHOLOGY 



Embryo. In all three genera of the Gnetales the embryo is dicotyle- 

 donous but its development varies. In Ephedra the division of the fer- 

 tilized egg nucleus results in the formation of eight free nuclei, around 

 each of which there is organized a cell that becomes an independent pro- 

 embryo (Fig. 308A). Only one or, rarely, two of these reach maturity. 

 The proembryo sends out a suspensor tube, at the tip of which a cell is cut 



Fig. 307. Male gametophyte of Ephedra trifurca. A, pollen grain with two prothallial 

 nuclei (p), generative nucleus (g), and tube nucleus (t); B, division of generative nucleus; 

 C, shedding condition with two prothallial nuclei, stalk nucleus (s), body nucleus (b), and 

 tube nucleus; D, pollen tube with tube nucleus (0, stalk nucleus (s), and two male nuclei 

 (m); A, B, C, X 1,500; D, X500. {After Land.) 



off. This gives rise to the embryo (Fig. 3085). In Welwitschia and 

 Gnetum the fertilized egg behaves as one of the proembryonal cells of 

 Ephedra, the embryo arising from the fertilized egg without any free- 

 nuclear division. This condition is characteristic of angiosperms. 



Summary. The occurrence of vessels in the secondary wood, of com- 

 pound staminate strobili, and the prolongation of the inner integument of 

 the ovule into a micropylar tube are unique features that distinguish the 

 Gnetales from all other gymnosperms. The presence of true vessels is an 

 angiosperm character. Others include the elimination of archegonia and 

 formation of free eggs (except in Ephedra), the elimination of free-nuclear 

 division in the embryogeny (except in Ephedra), the formation of two 



