3 66 



THE ANATOMY OF WOODY PLANTS 



by the accompanying figure (Fig. 264) of the stem of a seedling 

 collected by Professor Thompson in Southwest Africa. It is clear 

 that the same polydesmic organization of the axis is present as 

 is found in the older stem of species of Gnetum. In the South 

 African genus, however, the polydesmic condition extends to the 

 roots and is accordingly to be regarded as a more innate feature 

 of organization than in Gnetum. The structure of Welwitschia 



suggests a climbing 

 ancestry. Although 

 we do not know the 

 explanation of the 

 polydesmic condition 

 in climbers, it is 

 clearly co-ordinated 

 with the vine habit. 

 We may suppose that 

 the forbears of the 

 genus were originally 

 forest climbers and 

 that the surviving 

 strongly truncated 

 desert species has per- 

 sisted in its present 

 habitat with the re- 

 tention of the ances- 

 tral polydesmy. A similar suggestion has been made in an earlier 

 chapter as a possible explanation of the phenomenon of polydesmy 

 in the cycadean forms, living and extinct. 



Having considered the general topography of the stem in the 

 Gnetales with particular reference to the presence of large rays 

 of the dicotyledonous type and the phenomenon of polydesmy, we 

 may now profitably turn our attention to the more minute organ- 

 ization of the wood. The ligneous tissues consist of rays and 

 longitudinal elements. The former have already been sufficiently 

 discussed in previous paragraphs. The longitudinal structures 

 of the xylem consist of tracheids, vessels, and storage parenchyma. 

 The tracheids do not need any extended reference, as they have 



FIG. 264. Transverse section of young stem of 

 Welwitschia mirabilis. 



