STEMS, ROOTS, AND LEAVES 107 



production of new secondary phloem and xylem in these regions. 

 This new phloem and xylem, which is common in the older 

 portions of most herbaceous stems, is composed of the same 

 kinds of tissue elements as those produced by the cambium 

 in woody stems. Some of the tissue elements for example, 

 ducts and wood rays have certain features characteristic of 

 herbaceous stems, but these differences are not significant for 

 our general treatment. 



It is also evident that no annual rings of wood can be pro- 

 duced in an annual herbaceous stem like Salvia. The above dis- 

 tinctions between Salvia as a type of herbaceous stem and the 

 woody stems of trees previously studied may be taken as gen- 

 erally applicable to annual herbaceous plants and perennial tree 

 types. It should be remembered, however, that between the 

 two extremes all gradations in stem structure occur, so that it is 

 impossible to establish distinctive types of herbaceous and woody 

 stem structure. 



We have indicated above that in some herbaceous plants the 

 vascular strands remain isolated during the entire seasonal life of 

 the plant. In Salvia and its relatives there always exists a com- 

 plete cylinder of tissue, including the vascular- ^strands, which 

 corresponds to the thick vascular cylinder of trees. In larger 

 herbaceous plants, like the castor bean and the sunflower, a thick 

 vascular ring is developed, approaching the condition found in 

 shrubs like the roses, currants, and barberry. These latter plants 

 again present a graduated series in stem structure which bridges 

 the gap between the larger herbaceous types and the strictly 

 woody types of forest trees. 



The stem, like other parts of the plant body, is thus seen to 

 illustrate the principle of evolution in a complete series of closely 

 related forms. Some botanists now maintain that the course of 

 evolution in stems has been downward from the trees of early 

 geologic times to the herbs of the present day. If this is true, 

 then the vascular cylinder of Salvia is simply a greatly reduced 

 vascular cylinder of earlier tree forms, in which the mature stem 

 of the herbaceous type approximates in structure to young twigs 

 of a tree before the outer corky bark and the thick vascular 



