130 A TEXTBOOK OF BOTANY [Ch. IV, 3 



the cells are found well differentiated, as our picture illus- 

 trates (Fig. 85). Outside is the single layer of the epidermal 

 cells, with occasional stomata, not essentially different from 

 those in leaves. Just beneath lies the zone of collenchyma 

 cells, of which the thickened angles, elongated forms, and 

 composition from elastic cellulose explain their function as 

 the first strengthening tissue of the flexible and elongating 

 stems. Next comes the chlorenchyma, like that of the leaves, 

 though with scantier chlorophyll. Its innermost layer con- 

 tains starch, and constitutes the stabch sheath, of which 

 the function is disputed, some investigators assigning it a con- 

 ducting function for carbohydrates, while others consider 

 it a geotropic-perception sheath, as will later be explained. 

 Next inside comes a very prominent zone of angular, thick- 

 walled, light-colored, greatly elongated empty cells, found by 

 tests to be hard and stiff. These are sclerenchyma cells, 

 the characteristic strengthening cells of plants, found in 

 diverse situations, and here evidently giving special support 

 to the young stems of this vine, which stand out laterally 

 before twining around a support. 



Inside the sclerenchyma ring can be seen the fibro-vascular 

 bundles, which here present an unusually distinct structure. 

 Each bundle is ovate in cross section, with the point towards 

 the center, and shows three parts. The xylem, or wood, 

 inside of the cambium, contains the large, somewhat angled, 

 thick-walled ducts, lacking protoplasm but variously marked 

 by spirals, pits, and the like. They are formed by the 

 union of many cells of which the intermediate walls have 

 been absorbed. Intermingled therewith are smaller cells, 

 partly wood fibers and partly wood parenchyma, having 

 minor functions in connection vnth the conduction and 

 storage of carbohydrate foods. The characteristics of the 

 xylem arc further well illustrated in the diagrammatic 

 figure 86. 



The phloem, or bast, is composed of small thin-walled 

 elongated protoplasm-containing cells lying outside of, and 



