8 4 



GROWTH AND WORK OF PLANTS 



Fig. 74- 



Cross section of leaf of wintergreen. 

 CM., cuticle; Epid., epidermis; v.d., vascular 

 duct; In/, c. sp., intercellular space; L. ep., 

 lower epidermis; St., stoma. 



When viewed from the edge, as seen in a cross section of the leaf 

 (fig. 74), they appear quite regular and rectangular. 



141. The cuticle. Upon the outside of the epidermal layer 

 of cells is a more or less thickened deposit of a waxy nature, 



the cuticle. This is extremely 

 thin in some plants (shade plants, 

 especially in moist regions), while 

 in others it is quite thick, as in 

 the cabbage (and in plants of dry 

 regions). When the cuticle is 

 highly developed, as in the cab- 

 bage plant and onion, it is diffi- 

 cult to wet the leaf, since the 

 water rolls off so easily from the 

 smooth, waxy surface. 



142. The stomates. When 

 the epidermis is viewed from the 

 surface, as in fig. 73, here and 

 there are seen peculiar and quite regular cells in pairs surround- 

 ing a minute opening in the epidermis. This is a stomate, and 

 the cells which surround the minute opening are the guard cells. 

 In profile view each guard cell is nearly semi-circular; the tw r o 

 fitting together form a subcircular or subelliptical figure. When 

 seen in a cross section they are quite different in form, being 

 somewhat rectangular with an irregular outline next the opening. 

 Their form, structure, and relation to the surrounding cells of the 

 epidermis is such that in dry periods the stomates often close and 

 check the loss of water by the plant. Under such conditions the 

 guard cells lose part of their turgidity, and their form so changes 

 that their inner walls touch and close the opening. When the 

 plant has an abundance of water the guard cells absorb enough to 

 make them turgid, and, in swelling, their form and the thickness 

 of the walls causes them to arch away from each other and open 

 the stomate. 



143. Epidermal outgrowths, hairs, glands, etc. Leaves 

 are either smooth, or hairy, or rough from other outgrowths of 



