180 - BOTANY 



evaporation. By this transpiration from the aerial part of plants, 

 the water passing into them from the roots escapes, and at the same 

 time, by preventing saturation, which would otherwise be produced, 

 tends to maintain a continuous upward movement of the water. The 

 current of water thus produced is accordingly termed the transpira- 

 tion current. As the result of evaporation only water, in the form 

 of vapour, and gases can escape from the plant. As THE watery fluid 



ABSORBED BY THE ROOTS CONTAINS SALTS, OXIDES, AND OTHER NON- 

 VOLATILE SUBSTANCES IN SOLUTION, THESE ON EVAPORATION ARE LEFT 

 IN THE PLANT AND GRADUALLY INCREASE IN QUANTITY. This 



accumulation of mineral salts is absolutely necessary for the plant, for 

 the nutrient water taken up by the roots is so weak in mineral 

 substances (it contains but little more solid matter than good drinking- 

 water), that the plant would otherwise obtain too little food if it were 

 only able to take up as much water as it could retain and make 

 use of. 



All those contrivances in plants, therefore, which render 

 possible or promote evaporation, operate chiefly in the 

 service OF nutrition. Were transpiration not in the highest degree 

 useful and even necessary for the acquisition of mineral substances, 

 provision would certainly have been made by plants to restrict it 

 within the smallest possible limits. For transpiration increases the 

 amount of water required by plants disproportionally to their powers 

 of absorption, and exposes them, moreover, to the danger of perishing 

 through the insufficiency of their water-supply. 



Herbaceous land plants evaporate, in a few days, according to the calculations 

 of Sachs, more than their own weight of water. A Tobacco or Sunflower plant will 

 lose by evaporation in one day as much as a litre of water ; and it has been 

 estimated that trees lose, in the same way 50-100 litres daily. 



In spite of the increased danger of drying up, as the result of 

 evaporation, special provision is made by plants for facilitating trans- 

 piration (p. 188). To supply the increased demands for water thus 

 produced there is set up a strong current of water containing nutritive 

 salts in solution, which passes through the plants, and after yielding up 

 its solid constituents, escapes in the form of invisible aqueous vapour. 

 Thus plants, in order to obtain their nutrient substances, proceed in 

 the same manner as the smaller animals (Sponges, Ascidians), which 

 draw in and maintain a continual flow of water through their bodies, 

 in order to retain as food the nourishing particles suspended in it. 



The Absorption of Water. — "Water," as here used, it must 

 always be remembered, does not mean chemically pure water, but rather 

 a dilute watery solution of various SUBSTANCES IN the atmo- 

 sphere, FROM THE MINERAL SALTS OF THE EARTH, AND FROM ORGANIC 

 HUMUS. In this connection it is also necessary to emphasise the fact 



that LIVING PLANTS DO NOT ABSORB THIS NUTRIENT WATER IN- 



