GNETALES 393 



containing free nuclei or cells, and the functioning of both cells dis- 

 charged by a tube suggest the so-called "double fertilization" 

 characteristics of angiosperms. The peculiarity of the angiosperm 

 situation, however, is not that two male cells fuse with two eggs, 

 but that one of them enters into a triple fusion which results in the 

 so-called endosperm tissue. 



4. The embryo 



The development of the embryo of Gnetales differs widely from 

 that of other gymnosperms; and the three genera differ so much 

 among themselves that a separate account must be given for each. 



In 1872 Strasburger (3) described the remarkable embryo- 

 formation of Ephedra altissima, outlining a general situation which 

 has proved to characterize all the species investigated (fig. 445). The 

 principal features are as follows: free nuclear division up to eight 

 nuclei; the organization of two to eight independent cells; the inde- 

 pendent functioning of each proembryonal cell in the formation of a 

 suspensor and an embryo. The free nuclear division up to eight 

 nuclei repeats the early stage of the proembryo of most conifers; 

 but the difference appears in the separate organization and function- 

 ing of the eight cells, resulting in a proembryo of eight independent 

 cells rather than one consisting of a tissue of eight cells. Each func- 

 tioning proembryonal cell elongates to form a suspensor-like tube, 

 which emerges from the egg, penetrates the endosperm, and cuts off 

 at its free tip an embryo-forming cell. This is quite like the embry- 

 ogeny of conifers, except that several suspensors emerge from the egg 

 in various directions, rather than a single one emerging from the base 

 of the egg. It must be remembered, however, that among conifers 

 the constituent cells of a suspensor may separate, and an embryo 

 be formed at the tip of each one. This is exactly the case of Ephedra, 

 for its proembryonal cells act independently in embryo-formation. 



Fuller details of the embryogeny of Ephedra have been given by 

 Land for Ephedra trifurca (23). The behavior of the jacket cells 

 and of the non-functioning male nucleus, during the development 

 of the proembryo, is worthy of mention. Soon after fertilization, 

 the extremely thin walls of the jacket cells disappear, and the con- 

 tents of these cells become mingled with those of the egg. Shortly 



