234 THE MOVEMENTS OF WATER 



PART II. 



h > 



THE EXCRETION OF WATER VAPOUR 1 . 



SECTION 38. The Influence of Specific Peculiarities upon 

 Transpiration. 



All sub-aerial organs give off water- vapour to a greater or less extent, 

 and indeed it could hardly be otherwise in organs which are adapted for 

 rapid gaseous exchange with the surrounding air. It is, however, necessary 

 that, according to the conditions under which the plant exists, transpiration 

 may be so regulated and controlled that turgidity can be maintained, for in 

 plants which are sensitive to drought the percentage of water must never 

 fall below a certain minimum (Sects. 25, 27, 33). 



Many plants are compelled to use the little water they can obtain in 

 the most economical manner possible, and in such cases adaptations to protect 

 them from excessive transpiration are most markedly developed. Indeed, 

 the special shape and structure of typical xerophilous plants have mainly 

 this importance, for in order that they may cope with the conditions under 

 which they exist, the surface-area is reduced as far as possible, although 

 this places the plant at a disadvantage in other ways. Thus the regulatory 

 diminution of transpiration which becomes necessary when the supply of 

 water is limited involves a hindrance to gaseous exchange, and thus pre- 

 vents the full functional activity of the chlorophyll-apparatus from being 

 exercised (Sects. 29 and 30). 



To insure the harmonious co-operation of the whole, mutual conces- 

 sions of the most varied character are everywhere necessary, so that, for 

 the general good, peculiarities are often developed which may to a certain 

 extent hinder particular functional activities. This is the case with the 

 factor of transpiration with which a terrestrial plant has to reckon, and it 

 makes certain sacrifices in order that a purposeful regulatory control of 

 this function may be possible, although such modification frequently causes 

 the death of plants which have been transported to unsuitable habitats. 

 Transpiration is not a mere useless or even burdensome physical necessity, 

 but on the contrary it is of considerable physiological importance when 

 properly regulated 2 . 



Thus the rapid distribution of dissolved substances is in a large 

 measure due to the transpiration current (Sects. 22 and 35), and it is 



1 Burgerstein has given a complete summary of the literature (Materialien zu einer Monographic 

 d. Transp. d. Pflanzen, 1887, Th. i, and 1889, Th. ii, Sep.-abdr. a. d. Verhandlungen d. Zool.-bot. 

 Ges. in \Vien). 



8 Cf. Stahl, Bot. Zeitung, 1894, P- H 1 - The importance of transpiration was shown by 

 Boussingault, Die Landw. in ihrer Beziehung znr Chemie u. Physik, 1884, Bd. i, p. 20. 



