IQO THE ANATOMY OF WOODY PLANTS 



in turn abutting directly on the fibrovascular cylinder. The 

 latter is circular in outline and shows no variation of structure in 

 different segments. In b an annual branch of the sage is diagram- 

 matically represented so as to indicate the essential features of 

 organization. The stem is now square in outline, as is the usual 

 condition in herbaceous representatives of the Labiatae. It is to 

 the flat surfaces of the quadrilateral stem that the paired leaves 

 are attached. An examination of the fibrovascular cylinder shows 

 that the uniformity of structure characteristic of the round peren- 

 nial region is no longer present. Opposite the flat surfaces the 

 cylinder is much thinned out, and vessels are conspicuous by their 

 absence, except in small clusters marking the position of the leaf 

 trace, which will pass outward at a higher node. Although the 

 fibrous modification of the structure of the wood in relation to the 

 foliar fibrovascular strands is found here as in the case of Potentilla 

 and Alnus, it seems clear that we have to do with a less primitive 

 phase of the origin of the herbaceous type. In c is presented a 

 diagram of the extremely herbaceous stem of Lamium album. 

 The situation as regards flattening of the stem in connection with 

 the quadrangular structure characteristic of the axes in the labiates 

 is virtually the same as in the annual shoots of the sage, with the 

 difference that rays are no longer seen in the purely fibrous and very 

 narrow portions of the cylinder facing the flat surfaces and flanking 

 the leaf traces on either side. In still softer and more herbaceous 

 stems the fibers in turn may be largely or entirely replaced by 

 parenchyma, so that the cylinder becomes definitely broken up 

 into separate strands. Similar conditions in a general way present 

 themselves in all cases in which the axial organs of dicotyledons 

 become annual and assume the herbaceous texture for example, 

 the Solanaceae, as illustrated by the woody stem of the bittersweet 

 (Solanum Dulcamara) and the angular soft one of the potato 

 (S. tuber osum) and of the tomato (S. esculentum). In the two last- 

 named species the furrows of the stem as well as the fibrous or 

 parenchymatous regions of the fibrovascular cylinder correspond 

 in position to the leaves which are a factor in the evolutionary 

 processes under consideration. The Labiatae and Solanaceae 

 have been chosen to illustrate the development of the herbaceous 



