368 MORPHOLOGY OF GYMNOSPERMS 



secondary wood-formation, so that the vascular cylinder does not 

 become thick. The tracheary vessels are found in the inner region 

 of the cylinder, and consist of broad vessels associated with ordinary 

 gymnospermous tracheids. They are interrupted by oblique walls, and 

 have bordered pits as well as simple pits, suggesting an intermediate 

 condition between the characteristic gymnospermous tracheids and 

 the true vessels. A peculiar anatomical feature of the genus is the 

 diaphragm-like plate of cells which occurs at the base of each inter- 

 node, rendering the stem easily separable at the nodes. The stomata 

 occur upon the fluted stem in rows, and are sunken in urn-shaped 

 depressions formed by mounds of cuticle. 



In Welwitschia the vascular system of the stem is in a very con- 

 fusing condition on account of the shortened axis. At the junction of 

 the crown (the excessively shortened stem) and the huge tap root there 

 is a broad plate of collateral bundles, made up of two plates in con- 

 tact. The vessels of the upper plate connect with the leaves and the 

 strobili, and those of the lower plate with the system of vessels in the 

 tap root. The primary cambium is short-lived, and irregular series 

 of bundles suggest the existence of successive cortical cambiums, but 

 the testimony is too vague to use in any general conclusion. It is 

 reported that tracheids with bordered pits are entirely lacking, being 

 replaced by simple pitted and spiral vessels, which suggests a further 

 stage of advance in this feature than that exhibited by Ephedra. In 

 connection with the vascular bundles there is a conspicuous develop- 

 ment of fibrous sclerenchyma. A remarkable anatomical feature 

 of the genus is the occurrence of the so-called "spicular cells," which 

 are found in large numbers and are scattered throughout all the organs 

 (figs. 410-412). They are very large cells, fusiform or branched, 

 straight or variously curved, with greatly thickened walls, and often 

 so closely packed and interlaced that they make the tissues extremely 

 hard and tough. In the outer layers of the walls, and especially on 

 the surface of the longer spicular cells, crystals of calcium oxalate are 

 very conspicuous. 



In Gnetum the primary cambium is also short-lived, and successive 

 cortical cambiums produce concentric series of vascular bundles. 

 This common feature of Gnetum and Welwitschia and its occurrence 

 among the Cycadales is a fact of interest rather than of present sig- 



