172 COLLEGE BOTANY 



through the exposed parts of the plant. This process may also 

 be demonstrated by the use of the potometer, but it should be 

 remembered that an injured plant is abnormal and therefore its 

 activity is not altogether comparable to the uninjured plant 

 (Figs. 98 and 99). 



A very simple potometer may be made by using two glass 

 tubes, one of which is graduated. These two tubes are held in 

 an upright position and attached at their lower ends by a rubber 

 tube. They are filled with water and a freshly cut leafy shoot 

 fastened in the upper end of the ungraduated tube by means 

 of a perforated cork and sealed with wax or adhesive tape, so 

 that the water cannot escape except through the shoot. The 

 fall of the water in the graduated tube can be readily measured. 

 In nature, excessive transpiration results in wilting and 

 sometimes in the death of the plant. Cut flowers wilt because 

 they do not have a. water supply, although transpiration con- 

 tinues. If kept with the cut ends in water, they continue to 

 absorb water and maintain their rigidity. Apple, potatoes and 

 fleshy roots, although protected by heavy epidermal structures, 

 continue to transpire and slowly wilt when kept in storage. 



The Plant Mechanism Permitting Transpiration. We have 

 already learned that the structure of most leaves consists of an 

 upper and a lower epidermis of one or occasionally more layers 

 of cells in which the exposed surfaces are protected by a cuticle; 

 a layer of palisade cells next to the upper surface; a more or 

 less loose mass of mesophyll cells between the palisade and the 

 lower epidermis, and many stomata which are usually most 

 abundant on the lower surface. The stomata open into more 

 or less extensive passages or intercellular spaces among the 

 mesophyll cells (Figs. 59 and 60). The mesophyll cells in con- 



