GNETALES— WELWITSCHIA 



389 



sections of a thick Scotch plaid blanket as to try to cut a stem of 

 Welwitschia without imbedding. The dry wood is light and not hard, 

 but a treatment of 2 or 3 weeks in 20 per cent hydrofluoric acid should 

 precede any attempt at imbed- 

 ding. With such treatment Dr. 

 La Dema M. Langdon cut 

 very satisfactory thin paraffin 

 sections, 3 cm. square, includ- 

 ing the leaf groove, of a plant 

 15 cm. in diameter. 



Except in the leaf, the ar- 

 rangement of the bundles is ir- 

 regular. Occasionally, in 

 younger stems, and especially 

 in roots, there is a zonation, 

 doubtless representing growth- 

 rings; but it is doubtful wheth- 

 er such rings are annual. It is 

 more likely that they were 

 formed some season when 

 there was a heavy rain. We 

 have already noted that the 

 normal rainfall in the Wel- 

 witschia country is about i 

 inch; but there have been 

 floods. 



A median longitudinal sec- 

 tion of a stem, with a crown 

 15 cm. in diameter, shows a 

 cup-shaped plate of vascular 

 tissue about as thick as the 



leaf groove, and extending from one leaf groove to the other. This 

 is the principal vascular system from which bundles extend upward 

 to the crown and inflorescences, and downward to the stock and 

 root. 



Although the bundles are collateral, endarch, with a cambium, the 

 xylem of any individual bundle has no recognizable growth-rings. 



Fig. 367. — Welwitschia mirabilis: spicular 

 cells from the perianth of the staminate 

 flower: A, branching cell, the branching 

 represented in only one plane; B, a nearly 

 straight cell; C, transverse section showing 

 lumen nearly closed and surface incrusted by 

 small crystals of calcium oxalate; X22S. — 

 From Coulter and Chamberlain, Mor- 

 phology of Gymnos perms'^'* (University of 

 Chicago Press). 



