116 MORPHOLOGY OF SPEEMATOPHYTES 



ers, being among the prominent lianas of tropical forests (Fig. 

 85). The foliage is leathery in texture, and is very suggestive 

 of Dicotyledons, the well-developed opposite leaves being lan- 

 ceolate to ovate in outline and pinnately net-veined (Fig. 88). 



THE STEM 



Ephedra. — The characteristic long-jointed and fluted green 

 stems have been spoken of above. In the stem tip the dermato- 

 gen and periblem cap the plerome cylinder with distinct layers, 

 each layer having its own initials, or the three embryonic regions 

 merge in a common group of initials. - It is of interest to note 

 that both of these conditions have been found in a single species 

 (-£/. campylopoda) . There are no cauline bundles, as in Tum- 

 hoa and Gnetum, and no secondary cambium. The primary 

 cambium develops a certain amount of secondary tissue, but the 

 increase in diameter is never striking. The tracheary vessels 

 of the secondary wood, which distinguish Gnetales from other 

 Gymnosperms, are found in Ephedra at the inner part of the 

 secondary xylem cylinder, and consist of broad vessels associated 

 with ordinary gymnospermous tracheids. These vessels are in- 

 terrupted, moreover, by oblique walls, and display bordered pits 

 as well as simple pits. In every way they seem to be modified 

 gymnospermous tracheids. A peculiar feature of the genus is 

 the diaphragmlike plate of cells which occurs at the base of each 

 internode, rendering the stem easily separable at the nodes. 

 The stomata occur upon the fluted stem in rows, as in Equisetum. 

 and are sunken in urn-shaped depressions formed by mounds 

 of cuticle. 



Turnboa. — The anatomy of the stem is somewhat confusing 

 on account of the extremely shortened axis. In the region 

 where the short crown and large tap root merge there is a broad 

 stratum of collateral bundles, which De Bary recognizes as two 

 layers very close together, and calls the " layer of the leaf trace." 

 The upper layer connects with the numerous veins of the leaves, 

 and the lower layer is associated with the system of vessels in 

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

 of irregularly concentric bundles suggests a succession of sec- 

 ondary cortical cambiums. The resemblance to Cycads is fur- 

 ther marked by the occurrence of cauline bundles, not onlv in 

 the cortex but also in the pith. It is reported also that ti'acbcids 



