LEAVES 565 



above cases seems to be that the ancestors of the present forms lived in conditions 

 favorable for stomatal development, and that only the vestiges of such organs now 

 remain. Stomata also occur in various plants without chlorophyll, on some sub- 

 terranean organs, and on anthers and the interior parts of carpels, vi^here no relation 

 to carbohydrate synthesis is to be looked for. Such stomata either are functionless 

 or they may facilitate respiration and transpiration. It may be noted that some 

 subterranean stomata exhibit guard-cell movements. 



5. PROTECTION FROM EXCESSIVE TRANSPIRATION 



The significance of transpiration. — The importance of water. — Water 

 plays a dominant part in the life of plants, being the most important 

 single factor in determining the varying vegetation of ponds, deserts, 

 meadows, rock clifiEs, and of many other habitats. Water forms a large 

 part of the raw material from which plants build up foods and tissues, 

 and all plant activity depends upon its presence in considerable amount 

 in the cell sap. Although there is great and continual use of water, and 

 although the supply frequently is inadequate, there is an enormous and 

 increasing loss by transpiration. A large sunflower plant is said to 

 transpire a liter of water on a warm day, and a tree transpires many 

 liters. In many plants from 200 to 400 grams of water are said to 

 evaporate for every gram of dry solid matter produced. 



Transpiration and the absorption of salts. — Sometimes it is held that 

 all plant activities necessarily are beneficial, else they would have been 

 lost in the progress of evolution. Hence, various attempts have been 

 made to discover the advantage of transpiration. The chief theory has 

 been that, by accelerating the movement of water through plants, 

 transpiration increases the amount of available mineral salts, since they 

 enter and traverse plant tissues in solution in the water.' This theory is 

 not tenable, inasmuch as salts are not swept along in the water, but 

 enter plants and move from cell to cell independently of one another 

 and of the rate of movement of the water. The rate of entrance of a 

 salt depends usually upon the rate at which it is utilized within the 

 plant, since it moves from a place of high to one of low concentration. 

 Within the dead conductive vessels a rapid movement of the water may 

 facilitate the movement of salts, but no such phenomenon can occur in 

 the living cells of the roots and leaves (see p. 693). Furthermore, this 

 theory greatly exaggerates the amount of mineral salts used in plant 



• It has been believed even that the trembling of the leaf accounts for the rapid growth 

 of the aspen tree, since the increased transpiration accelerates water conduction, thereby 

 supposedly increasing the supply of salts. 



