366 



GYMNOSPERMS 



plants of Ephedra Irifurca developing in this way. Several conifers 

 are reproduced by underground buds. 



The seedling has a strong tap root, which persists for a long time, 

 but is gradually overtaken by adventitious roots. Root hairs are 

 abundant, but there is no mycorrhiza. 



The leaves are small and rudimentary, and, even when young, are 

 of scarcely any importance in the vegetative economy of the plant. 



They are opposite or in whorls of three, 

 very rarely of four. The leaves of Ameri- 

 can species are usually in whorls of three. 

 In all cases the whorls alternate. 



There is hardly anything which could 

 be called a blade. A thick and often fleshy 

 midrib has a thin border, thicker and 

 sheathing at the base, and with scarcely 

 any stomata. The border, at first whitish, 

 soon becomes brown, and withers. An ab- 

 scission layer develops early, and the leaf 

 falls ofT, leaving a broad scar. The leaf 

 trace is double, and extending into the leaf 

 is its only vascular feature. The double 

 trace does not extend beyond the node 

 above its insertion. In both leaf and stem 

 the trace is endarch. 



Histology. — At the apex of the stem and 

 root there are no separate initials for plerome, periblem, or dermato- 

 gen, and in the root there is probably no dermatogen at all. A 

 general meristem difTerentiates later into the body regions. 



The most striking histological feature of Ephedra, and of the other 

 two genera, is the presence of vessels in the secondary wood (tig. 346) . 

 The vessels are modified tracheids of all sizes, from that of the ordi- 

 nary tracheid up to several times that diameter, the largest being 

 formed early in the spring, smaller ones later, and still later in the 

 season none at all. The pits, at first bordered, enlarge and lose both 

 torus and border, so that they become mere perforations. Such per- 

 forations often fuse, and when they fuse on the oblique end walls the 

 structure becomes more and more like the continuous tube of the 



I'"iG. 347. — Ephedra Irifurca: 

 longitudinal radial section of 

 stem about i cm. in diam- 

 eter, showing one large vessel 

 and several tracheids; X170. 



