NATURE OF PLANTS 25 



ing possible vigorous photosyntheses would result in extensive 

 growth and leaf development. An extensive leaf area, however, 

 may not necessitate a heavy transpiration; the amount of water 

 given off will depend upon external conditions. In warm moist 

 climates conditions are favorable for photosynthesis but not for 

 transpiration, which is reduced in amount by the moist air. 

 So growth is vigorous, the largest leaves are developed and 

 transpiration is feeble. In warm but dry climates, on the other 

 hand, while conditions are favorable for photosynthesis the dry 

 air promotes excessive transpiration. This interferes with 

 photosynthesis, consequently very small leaves develop and the 

 volume of water transpired is correspondingly reduced. Under 

 conditions of moderate temperature and moisture as in most 

 temperate climates, the leaf assumes a size intermediate between 

 these two extremes because these conditions favor vigorous 

 photosynthesis and the resulting growth is not materially checked 

 by transpiration. So we see that the extensive transpiration in 

 temperate regions is in part the result of the large leaf develop- 

 ment and that this leaf area is dependent upon a vigorous photo- 

 synthesis. 



Only a small part of the water absorbed by the plant is re- 

 quired to furnish the necessary mineral substances and the water 

 utilized in its growth. The surplus is forced out of the cells and 

 finds its way in the form of vapor along the intercellular spaces 

 in the spongy mesophyll through the stomata to the air. The 

 transpiration of these large volumes of water must be of impor- 

 tance in keeping down the temperature of the plant during the 

 burning summer heat. Transpiration is often compared 'to the 

 evaporation of water from a dish. While it is controlled to a 

 limited extent in the same manner as evaporation, it should be 

 borne in mind that the giving off of water is intimately associated 

 with the vital activities of the cells and that the loss of water is 

 to a degree under the control of the plant. The cuticle, which is 

 practically impervious to water and gases, extends as a coat over 

 all parts of the plant body, and materially assists in controlling 

 the amount of transpiration. The thickness of this protective 

 coating depends upon external conditions, being developed in 

 3 



