216 IMPORTANCE OF WATER TO THE LIFE OF A PLANT. 



5. ABSORPTION OF WATER. 



Importance of water to the life of a plaut — Absorption of water by Lichens and Mosses, and by 

 Epiphytes furnished with aerial roots — Absorption of rain and dew by foliage-leaves — Develop- 

 ment of absorptive cells in special cavities and grooves in the leaves. 



IMPORTANCE OF WATER TO THE LIFE OF A PLANT. 



In the building up of the molecules of sugar, starch, cellulose, fats, and acids, of 

 proteids, and, in short, of all the important substances of which a plant is composed, 

 atoms of water have to be incorporated as constructive material, and without water 

 no growth or addition to the mass of a plant whatsoever could take place. From 

 this point of view water must be considered just as indispensable an item in the food 

 of plants as the carbon-dioxide of the air. But water plays, in addition, another 

 important part in plant-life. The mineral salts which serve to nourish hydro- 

 phytes, land-plants, and lithophytes, as also the organic compounds which are the 

 food of saprophytes and parasites, can only reach the interior of plants in the form 

 of aqueous solutions. They can only pass through a cell-wall when it is saturated 

 with water, and, having reached the interior of a plant, they can only be convej-ed 

 to the places where they are worked up through the medium of water. In con- 

 nection with the discharge of these functions in a living plant, water must be 

 regarded as a dynamic agent. Just as a mill on a stream only works so long as its 

 wheels are kept in motion by the water, and stops at once if the latter fails, or Hows 

 by in insufficient quantity, so the living plant, as it nourishes itself, grows and 

 multiplies, needs a continuous and abundant supply of available water to render 

 possible the performance of the complicated vital processes within it. This avail- 

 able or organizing water is not in chemical combination like that which is present 

 as food-material, and is, in general, not permanently retained. On the contrary, we 

 must conceive it as perpetually streaming through the living plant. In the course 

 of a summer, quantities of water, weighing many times as much as the plant itself, 

 pass through it. The total amount of water in chemical combination in the organic 

 compounds of a plant is very trifling compared with this, though it often happens 

 that the weight of the latter in a particular plant is greater than that of all the 

 other substances put together. 



Inasmuch as this water evaporates from plants in dry air, and that it may also 

 easily be withdrawn by alcohol or other means, very simple experiments suffice to 

 give an idea of the great bulk of free water in any plant. Berries, fleshy fungi, 

 succulent leaves, and things of that kind, if left in alcohol, are reduced in a short 

 time to barely half their size in the fresh state. The Nostocineaj, which are gela- 

 tinous when alive, and many fungi {e.g. Guepinia, Phallus, Spathularia, Dacryo- 

 myces) shrivel up so stringently in drying, that a piece possessing an area of 

 1 square centimeter when fresh leaves only a dry crumbling mass covering scarcely 

 3 square millimeters. A Nostoc, which weighed _ 224 grms. in the fresh state only 



