368 



GYMNOSPERMS 



nodes, giving the general impression of partitions in bamboos and 

 other grasses. 



The small, more or less upright, green stems, which give Ephedra 

 such a characteristic appearance, do the photosynthetic work. The 

 long-fluted internodes and reduced leaves make these small shoots 

 look like Equisetum, and their internal structure also resembles that 



of the ancient pteridophyte 

 (fig. 349). They grow from a 

 small meristem at the base, 

 and many of them fall off at 

 the end of the growing season, 

 thereby effecting a very advan- 

 tageous reduction in the trans- 

 spiration surface. In some 

 there is a vigorous develop- 

 ment of secondary wood, and 

 these, later, produce whorls of 

 small branches. Branches of 

 all sizes are shown in fig. 345. 

 The epidermal cells are 

 thick walled, with a group of 

 very thick-walled cells under 

 the ridges ; stomata arc distrib- 

 uted at the bottom or along 

 the sides of the depression be- 

 tween ridges. Between these 

 thick-walled cells and the vas- 

 cular region is a zone of very 

 thin-walled cells with abun- 

 dant chloroplasts and numer- 

 ous intercellular spaces. The 

 vascular cylinder is an endarch siphonostele. In the center of the 

 pith is a group of rather thick-walled cells with very dense contents, 

 perhaps tannin: there is no resin in the order. 



I'"iG. 349. — Ephedra trijiirca: transverse 

 section of a small upright green twig, about i 

 mm. in diameter, showing two ridges, the 

 depression between them, and a stoma at the 

 base of the depression. There are thick-walled 

 cells under the ridges, and a zone of chloro- 

 phyll cells extending down to the vascular 

 region; X270. 



THE SPOROPIIYTE — REPRODUCTIVE 



Ephedra, and also the other two genera, are dioecious, but in all of 

 them there are traces of an ancestral bisporangiate condition. By 



