THE STEM 



187 



advantageous. In Fig. 140^ is shown the stem of a woody Poten- 

 tilla (Potenlilla palustris). There are five broad segments of the 

 stem, and alternating with these are five narrower ones. The more 

 exiguous segments are related to the leaf traces and are charac- 

 terized, as are the similar regions of the woody axes of Casuarina 

 and Alnus repre- 

 sented in earlier 

 illustrations in this 

 work, by the con- 

 spicuous absence 

 of the vessels pres- 

 ent in the remain- 

 ing and broader 

 sectors. Although 

 in the outer region 

 the sectors under 

 discussion show a 

 complete absence 

 of vascular struc- 

 tures, in the region 

 near the pith a few 

 elements of this 

 nature are present. 

 These belong to 

 the leaf trace 

 proper, which lies 

 on the inner sur- 

 face of the narrower segments of the stem. Not only is the leaf 

 trace segment narrower and distinguished by the absence of vascular 

 structures, except in the actual trace itself, but it is depressed below 

 the surface of the adjoining broader sectors. In b these features 

 are indicated on a larger scale, so that the .conditions described 

 become the more obvious. It will be noted under the more favor- 

 able conditions of greater enlargement that the cluster of vessels 

 lying on the inside of the narrow segment and near the pith is 

 flanked on either hand by a region free from vessels. In c is shown 

 the organization of the perennial region of the stem in another 



A 



FIG. 140. Diagrammatic representation of stems of 

 species of Potcntilla. Explanation in the text. 



