3C4 Causes and Course of Organic Evolution 



"Within tliis is the "stylar cyUnder" of Treiib, which exactly 

 agrees witli the open stylar canal of Ephedra, except that it 

 is more intimately fused and reduced in Casuarina. 



In Ephedra a single orthotropous ovule is formed, and this, 

 according to the above interpretation of the "integument" 

 and descriptions hitherto given, would seem to be devoid of 

 s])ecial ovular integuments. The two ortliotropal ovules in 

 Casuarina arise in succession and develop each two ovular integ- 

 uments. So unless detailed study of other species of Ephedra 

 than those already examined reveal one or two ovular integu- 

 ments, it would seem as if these had been absorbed, or had 

 failed to develop in that genus. 



It ysiW thus be seen that a dimerous and decussate floral 

 symmetry is typical for both groups, as for Welwitschia and 

 Gnetuin. 



Passing to the ovular development, the studies of Land 

 for Ephedra and of Treub as well as Frye for Casuarina prove 

 that in both a patch of megaspore mother cells arises in the 

 megasporangial or ovular tissue. If the condition in E. trifurca 

 is typical for the genus, it would appear as if considerable 

 reduction had taken place as compared with Casuarina, for, 

 while four is the number in the former, an extensive patch 

 or plate is met with in the latter. It is worthy of note in this 

 connection that Gnetuni gnemon as described by Lotzy con- 

 forms to Casuarina. 



An ordinal difference however is now observed. For,, while 

 in Ephedra one megaspore — the innermost — enlarges to form 

 a single daughter megaspore cavity, Avhose nucleus divides 

 repeatedly to form several hundred megaspore daughter nuclei, 

 round which protoplasm accumulates and walls are formed to 

 constitute a gymnospermic prothallus, in Casuarina division 

 into numerous — 15-20 — complete cells takes place, each of 

 which becomes a megaspore or embryo-sac, while disorganiza- 

 tion of others around takes place. This would indicate that 

 in the evolution of the angiospermic Casuarina from the gymno- 

 spermic Ephedra the i^rothallial tissue of the former has divided 

 up into the several prothallial constituents or daughter mega- 



