134 OUTLINES OF PLANT LIFE. 



EXERCISE XXVIII. 



To show the variation in the rate of evaporation due to the difference 

 in structure of the organ. (See also 1[ 339.) 



Compare as shown by shrinkage or by loss of weight, (a) Through 

 cork tissue and without it. Take two potatoes ; peel one ; expose side 

 by side ; compare day by day. {b) Through skin. Compare in same 

 way two apples, {c) Through stomata. Take three equal leaves of 

 oleander ; of one close the stomata (which are on under side only) with a 

 thin coat of grafting wax, or cocoa-butter melted and brushed on (taking 

 care not to kill cells by having wax too hot) ; coat the upper surface of 

 second in same way ; leave third uncovered. Compare day by day. 



178. Amount and regulation. — The amount of transpira- 

 tion, therefore, varies with the structure of the leaf rather 

 than with its area. The temperature, percentage of moisture 

 in the air and movements of the air affect profoundly, the 

 rapidity of transpiration. Hence arises the need of regula- 

 tion by the plant, to prevent excessive loss. The guard cells 

 of the stomata are irritable, so that external conditions affect 

 their turgor. If both are turgid, they become curved away 

 from each other so as to increase the size of the opening be- 

 tween them. If they are flaccid, the thick ridges along the 

 inner face of each cell straighten them, and so close the 

 orifice more or less completely (fig. 105). The presence or 

 absence of hairs upon the leaves, the existence of stomata 

 upon one or both surfaces, the sinking of the guard cells 

 below the general leaf surface, the distribution of the stomata, 

 the thickening of the leaves, their inroUing (fig. 197), or 

 revolution, have a decided effect upon the rate of transpira- 

 tion, and may be adapted to regulate it. (See T^ 335 ff.) 



EXERCISE XXIX. 



To show that many leaves are not wetted by water. 

 Immerse various sorts of leaves in water. Does the water wet the 

 surface ? What is the cause of the silvery reflection of light from the 



