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- 
