GNETALES 



379 



the "perianth" persists and becomes lignified. In E. trifurca only 

 six months elapse between the first appearance of the o^a^late strobilus 

 and the germination of the seed. 



The development of the megasporangium of Welwiischia is un- 

 known, except that a group of mother cells is present beneath long rows 

 of parietal cells, the inference being that a hypodermal archesporium 

 of several cells is differentiated. 

 No pollen chamber is developed 

 in the nucellar cap, but it be- 

 . comes riddled by the descending 

 pollen tubes and the ascending 

 prothaUial tubes (fig. 424). In 

 fruit the enlarged bracts of the 

 strobilus become bright scarlet, 

 and the seed is winged by the 

 expanded perianth. 



The three envelopes investing 

 the nucellus of Gnetum have given 

 rise to much discussion, especially 

 as they are said to arise in acrope- 

 tal succession (4, 15). Beccari 

 (4) regarded the outermost one 

 as a perianth; Strasbtjrger (10) 

 and Celakovsky considered 

 them all as integuments, the 

 delicate middle one being only 

 an outer layer of the inner in- 

 tegument; while LoTSY (15) 

 recognized only the innermost 

 one as an integument. Usually 

 only ovulate flowers are found in ovulate strobili (fig. 425), but occa- 

 sionally staminate flowers appear. There are two kinds of ovulate 

 flowers (10): those of the ovulate strobili, as described above; 

 and those found with considerable regularity in the staminate 

 strobili. The latter differ in having only two envelopes, the middle 

 one of the complete flower being absent (fig; 426). These incom- 

 plete flowers usually do not function, but sometimes they develop 



Fig. 424. — WelwUschia mirabilis: 

 upper part of nucellus, with ascending 

 prothallial tubes; t, prothallial tubes; s, 

 sterile cells of fertile region of prothal- 

 lium; n, nucellus; m, megaspore mem- 

 brane; X140. — After Pearson (20). 



