216 IMPORTANCE OF WATER TO THE LIFE OF A PLANT. 
5. ABSORPTION OF WATER. 
Importance of water to the life of a plant—Absorption of water by Lichens and Mosses, and by 
Epiphytes furnished with aérial 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 conveyed 
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 flows 
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 Nostocinez, 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 
l square centimeter when fresh leaves only a dry crumbling mass covering scarcely 
3 square millimeters. A Nostoc, which weighed 2'224 grms. in the fresh state only 
