GNETALES 365 



but also the extreme condition found in strap-shaped lianae. 

 Fig. 263 portrays this condition in the stem of G. latifolium. The 

 woody strands of the polycyclic cylinder fail to develop on two 

 opposite sides of the axis, and this organization is correlated with 

 the flattened transverse section in the stem. It is obvious that 

 the genus under discussion has advanced to a condition of 

 organization such as is paralleled by perennial climbers among 

 the dicotyledons, and it must therefore be considered as having 

 reached a high evolutionary position. 



The broad short axis of the remarkable genus Welwitschia may 

 now claim our attention. Here the stem never attains a height 



FIG. 263. Transverse section of the flattened stem of Gnetum latifolium 



of more than a half-meter and bears two large perennial leaves 

 which, according to the investigations of Bower, are not the 

 persistent cotyledons, but a subsequent pair of foliar organs. The 

 perennial leaves of Welwitschia are inclosed at their bases in hollow 

 spaces resulting from the outgrowth of the stem. Within these 

 cavities, which function as moist chambers, are situated the basal 

 growing regions of the leaves; they are thus preserved from fatal 

 desiccation under the extremely arid conditions connected with 

 the desert habitat of the genus. Professor W. P. Thompson has 

 been able, through the co-operation of the Sheldon Foundation 

 of Harvard University, to secure an abundant supply of material 

 illustrating the anatomical organization of Welwitschia. When 

 the results of his investigations have been published, our knowledge 

 of this interesting and aberrant South African genus will be largely 

 increased beyond that supplied in Hooker's well-known memoir. 

 The general organization of the axis in the genus is well illustrated 



