326 PHYSIOLOGY 



A possible advantage. — There is only one region in the plant where 

 solutes may move with the water; that is, where solutions move as 

 a whole, namely, in the conducting tissue, which extends from root cortex 

 to leaf cortex. But solutions cannot enter this tissue in the live plant 

 without first passing through several live cells of the cortex, where os- 

 motic movement only is possible; nor can they usually reach the evap- 

 orating surface of a leaf (the wet walls of the aerating passages) without 

 passing several live cells, where again the solutes and water must move 

 independently. (See movement of water, p. 341.) It is conceivable 

 that the relatively rapid movement of solutions along this portion of the 

 path from root to leaf may be advantageous to the plant by placing a 

 greater supply of salts within reach of the leaves ; but there is no proof 

 that plants depend on this arrangement for an adequate amount of salts. 

 Moreover, this is rendered improbable by the fact that many plants grow 

 most luxuriantly with practically no transpiration for months at a time 

 to set up such a stream of solutions along the conducting tissue. 



A menace to life. — Transpiration, far from being a function of plants, 

 is an unavoidable danger. That it is a danger, a real menace to life, 

 is almost a matter of common observation. Millions of plants perish 

 annually because the outgo of water is greater than the income. A 

 loose soil and an exposed situation, sudden extreme evaporation due to 

 a hot dry wind and a blazing sun, or prolonged drought, are causes of 

 death only too well known to farmers in some regions. Scarcely a plant 

 escapes the loss of some parts by reason of shortage in the water supply; 

 and in temperate regions, with the average rainfall (say 100 cm. 

 annually), few plants attain the development of which they are capable 

 with a larger water supply. The luxuriant weed of well-watered ground 

 compared with the same weed, meager and dwarfed on the dry wayside, 

 illustrates what a menace to life and vigor is the evaporation from 

 plants. 



Transpiration and growth. — There are, of course, other causes of 

 stunting and meager development than transpiration. If some of these 

 operate to reduce vigor and growth, transpiration is affected thereby. 

 In fact, growth and transpiration, in seedlings at least, seem to be recip- 

 rocally related, and the one varies directly as the other, when an ample 

 supply of water is available, as in a water culture. It is not improbable 

 that a like relation exists under these conditions in mature plants. 



Transpiration unavoidable. — Dangerous as transpiration is, it is 

 unavoidable, because moist cell walls must be exposed to permit solu- 



