TRANSPIRATION 



43 



variations in external conditions, to modify those structural adaptations which 

 aim at aiding or retarding transpiration. This capacity the plant possesses during 

 its development only, it is true, within certain definite limits (Lecture XXX). 



From what has been said it will be seen that the amount of transpiration 

 must vary greatly in different plants, and in the same plant under different 

 conditions. Further, transpiration in individual organs varies very greatly. 

 [Under ordinary circumstances transpiration from the foliage leaves is so pre- 

 eminently greater than from other parts that we may really regard them 

 as the organs of transpiration, and in consideration of the extent of surface 

 they exhibit and the numbers of stomata they bear, this function is at once 

 obvious.] Transpiration in the individual organ varies also with its develop- 

 mental condition. Into these problems we need not enter here, but confine 

 ourselves rather to inquiring whether such variations are bound up with the 

 nature of the plant or whether they are accidental, in other words, whether 

 the enormous exhalation of water vapour seen in certain plants is useful and 

 essential to their welfare, seeing that others, especially submerged plants, can 

 do without transpiration altogether. The answers given to this question have 

 been by no means unanimous ; indeed some have been diametrically opposed 

 to each other one author holding that transpiration is a necessary evil (VoL- 

 KENS, 1887), another that it is vitally indispensable. Our experiences in these 

 latter days have taught us, however, that there is nothing more likely to lead 

 to error in the realms of physiology than the making of generalizations, since 

 in more than one aspect differences in the most fundamental vital conditions 

 have been shown to exist amongst organisms which outwardly give no hint 

 of them. Without anticipating a detailed exposition later on, reference may 

 be made here to certain lower plants, the condition of whose existence is a 

 medium free from oxygen and which stand out in sharpest contrast to ordinary 

 organisms which require free oxygen. It would certainly be quite wrong for 

 us to conclude that, because individual plants can get on without transpira- 

 tion, therefore transpiration was not essential to any. One thing is clear ; the 

 entire structure of the land plant necessitates transpiration, since, were it to 

 cease, the absorption and excretion of other gases would be impossible, and 

 the life of the plant would come to an end. Plants inhabiting dry climates 

 show us how far such a limitation of gaseous exchange can go. If we fail to 

 find such protective adaptations against excessive transpiration in the majority 

 of plants it must not be concluded that it would be impossible for these plants 

 to develop such adaptations ; on the contrary, we must hold that they have 

 no use for them. Finally, if we meet with plants which manifestly possess 

 adaptations for accelerating transpiration, we ought to conclude that tran- 

 spiration is a fundamentally important process in such plants. As a matter 

 of fact, this view may be substantiated in several ways. There can be no 

 doubt that transpiration markedly aids in the absorption from the soil, in large 

 quantities, of the salts whose significance we shall have to discuss in an early 

 lecture. [The influence of transpiration on the absorption of salts in the soil 

 is very clearly demonstrated by TREUB'S researches on the absorption of 

 potassium nitrate (1905, Annales Jardin bot. Buitenzorg, 2nd ser. 4, 119).] 

 These salts are presented to the roots in very dilute solution, and it would 

 take a very long time to transfer them to the highest branches of a tree by 

 diffusion only. In reality these salt solutions are transported by way of the 

 vascular bundles, and are carried right up to the cells of the leaf. Here eva- 

 poration takes place and a consequent concentration of sap and accumulation 

 of salts. Moreover, there is another effect of transpiration which must not be 

 overlooked. The leaves are exposed to sunlight and, since they are able to 

 absorb light rays by means of their chlorophyll and, in certain cases, by means 

 of other colouring matters also, they must become sensibly warmer. Obser- 



