THE TISSUES OF THE STEM 35 



through which the strands keep a separate course down the 

 stem. Such differences will naturally alter the number and relations 

 of the strands. But as they all regularly take the curved course in 

 the way shown in Clematis, the vascular strands will appear in all 

 ordinary Dicotyledons to be arranged in a circle in the transverse 

 section. Within that circle lies the central column of pith. Out- 

 side it is a more or less broad band of cortex, and externally is the 

 epidermis. The pith and cortex thus embed the vascular system, 

 and are sometimes called collectively the ground-tissue. It is in 

 herbaceous Dicotyledons that these tissues are most easily studied, 

 such as the Bean, Sunflower, or Potato. 



STEMS OF HERBACEOUS DICOTYLEDONS. 



The superficial layers of the stem in herbaceous Dicotyledons 

 show well-marked characters, and are relatively simple in construction 

 (Fig. 22). Starting from the outside, the epidermis appears as a 



FIG. 22. 



Superficial tissues of the stem of Potato. ( x 100.) ep = epidermis ; chl= outer- 

 most layer of the cortex containing chlorophyll corpuscles, coll = collenchyma ; end 

 = endodermis with starch grains. The protoplasmic contents of the cells are omitted 

 on the right, so as to make the cell-walls appear more prominent. 



single superficial layer of living cells, each with its lining of cytoplasm. 

 The outer wall of each cell is thickened, and covered externally by 

 a thin film of cuticle, which, being almost impervious to water, con- 

 trols the loss by evaporation. The epidermis thus forms a skin 

 protective both mechanically and physiologically. It is, however, 

 interrupted here and there by breathing pores, or stomata ; but none 



