How Substances are Transported and Removed 209 



Such are the principal facts as to transpiration, and they bring 

 us to the problem of its physiological meaning, upon which also 

 there is uncertainty. The older explanation argued thus: — 

 plants need in all parts, and especially their leaves, certain 

 minerals from the soil: their only possible method, apparently, 

 of raising these minerals to the places of use consists in absorbing 

 and transferring them in water, and evaporating the latter to 

 leave them behind : some of the minerals are so scarce that plants 

 hardly ever can get as much as they need: the more copious the 

 transpiration the more minerals are raised; presumably, there- 

 fore, transpiration is the mineral-raising process and is the more 

 efficient the more copious it is. On this assumption, plants would 

 be expected to develop adaptations for promoting transpiration, 

 and a great many such have actually been claimed to exist, as 

 will presently appear. A second explanation argues thus: — the 

 stomata exist primarily for admission of carbon dioxide needed 

 in photosynthesis (they occur, in general, only in green tissues) : 

 when open for this purpose, evaporation and diffusion of water 

 will necessarily take place from the saturated cell-walls of the 

 interior of the leaf as a purely physical operation which the plant 

 has no power to prevent: presumably, therefore, transpiration is 

 merely an incidental physical accompaniment of photosynthesis, 

 a kind of necessary evil, as it were. Upon tliis explanation 

 adaptations would be expected for its prevention, especially of a 

 kind which would not interfere with photosynthesis; and of these 

 a good many have been described, as we shall note in the follow- 

 ing chapter. This explanation accounts best for most of the 

 phenomena, and is the one that is generally accepted at present. 

 A third explanation argues thus: — w^hen full sunlight falls on a 

 leaf, it beats thereon with an energy overwhelmingly greater 

 than the leaf can employ in its work (for it actually uses no more 

 than some three per cent) : this energy, both light and heat, 

 would work disaster to the Uving protoplasm unless dissipated 

 in some manner: evaporation is a highly effective method of 



