GROWTH AND WORK OF PLANTS 



plants are seen to be very irregular in outline, as in fig. 74. When 

 viewed from the edge, as seen in a cross section of the leaf (fig. 

 75), they appear quite regular and rectangular. 



143. 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 

 cabbage plant and onion, it is 

 difficult to wet the leaf, since the 

 water rolls off so easily from the 

 smooth, waxy surface. 



144. The stomates. When 

 the epidermis is viewed from the 

 surface, as in fig. 74, here and 

 there are seen peculiar and quite 

 regular cells in pairs surrounding 

 a minute opening in the epider- 

 mis. This opening 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 two fitting together form a subcir- 

 cular 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 cause them 

 to arch away from each other and open the stomate. 



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



Fig. 75- 



Cross section of leaf of wintergreen (seen 

 under the microscope). CM., cuticle; Epid., 

 epidermis; v. d., vascular duct; Int. c. sp., 

 intercellular space; L. ep., lower epidermis; 

 St., stoma. 



