488 



RELATION TO ENVIRONMENT. 



stance there is a less surface for transpiration. The needle-like 

 or awl-shaped leaves of the conifers, junipers, etc., are examples 

 of xerophytic structures adapted to lessen transpiration. The 

 leaves of yuccas, while quite large, are very narrow and pointed in 

 proportion to the mass of the leaf. This kind of leaf is charac- 

 teristic of many plants of deserts or permanently arid regions. 

 Examples need not be multiplied here ; some are mentioned under 

 the general treatment of leaves (Chapter XL), and others in 

 treating of desert, arctic, heath, salt-marsh, and other societies. 

 Periodic and complete reduction of leaf surface takes place in 

 tropophytic plants in preparing them for the xerophytic stage 

 in which they pass the winter (see paragraph 938). 



.2. By protective covering or movements. Protective coverings 

 in the form of hairs, scales, a more or less thickened cuticle, 

 thickening of the epidermal wall, or a doubling of the epidermal 

 layers retard the loss of water. Protective movements of leaves 

 also take place in some plants, which tends to lessen transpira- 

 tion. A slight loss of turgor often causes leaves to droop and 

 assume a position in which transpiration is lessened. Remark- 

 able movements of leaves take place in the so-called sensitive 

 plants, belonging to the acacias and mimosas. These are highly 

 developed in arid regions. When the loss of water passes the 

 optimum, the leaflets fold together, or the whole leaf droops and 



brings the leaflets in close 

 approximation which retards 

 transpiration. The leaves of 

 many grasses become rolled 

 up and thus lessen the loss 

 of water. The leaves of the 

 evergreen rhododendrons roll 

 up in a striking way when 

 the loss of water becomes 

 Fi g excessive in drought, but es- 



Deeply sunken stoma of Franklandia peciallv in Very cold Weather, 

 fucifolia. (After Schimper.) * 



3. Action ana position o] 

 the stomata. The work of the stomata in regulating transpira- 



