218 TEXTBOOK OF BOTANY 



epidermis ( Fig. 48, page 91 ) . Turgor pressure in these cells is opposed 

 to turgor pressure in the cells in the lower side of the leaf. When turgor 

 pressure is high in these special cells the leaf is expanded. Leaves of 

 many kinds of plants without such special cells also exhibit various 

 degrees of rolling during periods of drought. 



The folding of leaves of clover, honey locust, and sensitive plant in 

 late afternoon or early evening is probably the result of unequal changes 

 in permeability or in digestion of starch to sugar accompanied by changes 

 in turgor and swelling of cells on opposite sides of the pulvinus (Fig. 

 47). These movements take place rapidly at higher temperatures, and 

 very slowly or not at all at lower temperatures or when the plant is 

 anaesthetized. 



Changes in permeability, turgor, and growth accompanied by conse- 

 quent curvatures and movements may occur in some plants as the result 

 of mechanical contact. The folding of leaves of the sensitive plant and 

 the twining of tendrils are familiar examples. Somewhat less familiar are 

 similar movements of the stigmas and stamens of certain plants, the 

 closing of the leaves of the Venus's fly-trap Dionaea, and the bending 

 of the tentacles on the leaves of sundew, Drosera (Fig. 77). 



Some of the proteins in the insects that are entrapped by such plants 

 are digested by enzymes from the plant or from bacteria. The resulting 

 compounds diffuse into the cells of the plant. The insectivorous plant, 

 however, is not necessarily dependent upon this external source of 

 nitrogen compounds. 



Stomates. The opening and closing of stomates are dependent upon 

 the turgor and relative wall extensibility of the two guard cells. When 

 they are expanded by increased turgor, unlike ordinary cells, tliey do 

 not become spherical. The inner wall that bounds the pore is thicker and 

 less stretchable than the outer wall away from the pore. Hence when 

 turgid, the outer walls stretch and the guard cells become curved or bean- 

 shaped. Since the concave sides of the two guard cells are adjacent, an 

 opening appears between them. When these cells contract with de- 

 creased turgor the concave walls straighten and come together; the 

 stomate is closed. This straightening of the concave walls may be due 

 to the contraction of the walls, but the pressure of the surrounding epi- 

 dermal cells also pushes the non-turgid guard cells together. If these 

 epidermal cells were to shrink excessively they might also pull the guard 

 cells away from each other. 



As a guard cell expands with increased turgor evidently the wall on 



