WETMORE: LEAF-STEAl RELATIONSHIPS 21 



illustrated. As is well known, this plant possesses a protostele in the stem. 

 The apical meristem is not flat as in most Angiosperms; the leaves are 



borne laterally (Fig. 5). There is a central procambial colunm — i.e., no pith 



which rises above the last leaf primordium. Cortical ground meristem also 

 exists above the highest leaf primordium. In other words, the stem now shows 

 a potential epidermis, a potential cortex, and conducting tissues without the 

 "influence" of leaf primordia. 



It may be argued that this aquatic plant is modified in relation to its envi- 

 ronment. Possibly so; nevertheless, it seems significant that the plant should 

 be developing at all, if the axis of Angiosperms is a system of phytons or 

 growth units only. On such slight evidence one can do no more than suggest the 

 possibility that Angiosperms have a shoot system, potentially both cauline and 

 foliar, in which ordinarily the leaves possess a dominant and the stems a 

 minor "influence" on development, but in which on occasion the stem may 

 hold the major role and the leaves a minor though necessary one. In this 

 connection one might refer to certain seeming cauline bundles — certainly not 

 associated with leaves — which Boke (1941) reports in the cactus Trichoccrcus 

 spachianits. 



It is instructive to examine other groups of vascular plants for develop- 

 mental patterns. Basing one's judgments on the Angiosperms alone may 

 well produce a limited outlook. May I call your attention to the genus Lyco- 

 podium in which our laboratories have been interested for some time. The 

 conclusions are based on a careful investigation of nine species. Those repre- 

 sentatives which we have studied from the Urostachys segregation of Lyco- 

 podiuni, L. lucidulum Michx. and L. Sclago L. have a flat-topped apical 

 meristem with erect foliar primordia (Fig. 6). The protostelic vascular cylinder 

 is forecast in a recognizable column of procambial tissue which rises higher 

 toward the apex than the place of origin of the youngest leaf primordium. 

 Young leaf primordia already show procambial strands related to them, never 

 discontinuous with the central column. The cortical ground meristem is 

 belated in appearance. There is never a pith nor is there any adaxial vacuolated 

 ground meristem with its associated leaf gap. By studying successive trans- 

 verse sections below the apex it is seen that the cauline vascular tissues — the 

 metaxylein and metaphloern — are outlined or blocked out, within 100/x of 

 the apex in L. lucidulum, whereas the first sign of the differentiation of pro- 

 toxylein occurred only about 300/x from the apex. However, though blocked 



Explanafion of figures 6 and 7 



Longitudinal sections of serial stem tips of Lycopodiuiii to show proximity of pro- 

 cambial column to the stem apex. Fig. 6. L. Sclago; stem apex flat-topped (X260). 

 Fig. 7. L. sabmaejoliuni; stem apex conical (X300). 



