356 



PLANT MORPHOLOGY 



united stamens that surround a sterile ovule (Fig. 305 A, C). Each sta- 

 men bears three terminal microsporangia forming a synangium. In all 

 three genera the scales at the base of each staminate flower are sometimes 

 designated as a "perianth." 



Ovulate Strobilus. The ovulate strobilus of Ephedra is simpler than 

 that of either of the other genera, consisting of an axis bearing several 



Fig. 303. Welwitschia mirahilis. Female plant in foreground; male plant in background. 

 {From a photograph furnished by the Chicago Natural History Museum.) 



opposite pairs of bracts and an erect terminal ovule or, in some species, of 

 two or more ovules (Fig. 302C). In Welwitschia and Gnetum the strobilus 

 has a long axis with many ovules borne in the axils of bracts (Figs. 304^ 

 and 305i^). In each genus an "ovulate flower" includes a single ovule 

 invested by a pair of scales that constitute a "perianth." The ovule has 

 two integuments, the inner one forming a long tubular micropyle (Figs. 

 305D and 306A). The outer integument has an outer fleshy layer and an 

 inner stony layer. There is a set of vascular strands in the fleshy layer of 

 the outer integument. In Ephedra a pollen chamber is formed by the 

 breaking down of some of the nucellar tissue but there are no swimming 

 sperms. In Gnetum, but not in Welwitschia, there is a slight tendency 

 toward the formation of a pollen chamber. 



Female Gametophyte. The female gametophyte differs greatly among 

 the three genera of Gnetales. In Ephedra it develops as in the Coniferales, 



