78 ABSORPTION OF FOOD-SALTS BY WATER-PLANTS. 
complicated adaptations are necessary for the extraction of food-salts from the 
earth. In particular, the portions which are exposed to the air above ground exhibit 
a number of special structures connected with this extraction. These structures 
(cuticle, stomata, &e.) are superfluous in the case of aquatic plants, for there is with 
them no necessity for raising and conducting food-salts into the parts where they 
can be used up. Moreover the absorption of nutritious matter is much simpler, 
Inasmuch as it is not necessary for the absorbent parts to search for a perpetual 
source of the requisite substances. The roots of land-plants have often to range 
over a wide area in order to find sufficient nourishment in the earth, and frequently 
they have then to liberate it, i.e. bring it into a state of solution. This is not the 
case with water-plants. They are completely surrounded by a medium which 
is itself to a large extent a solution of food-salts, and no sooner are substances 
withdrawn by the absorbent cells from the layers of water immediately bounding 
them than those substances are again supplied from the more remote environ- 
ment. Constant compensating currents occur in water, and there is, therefore, 
scarcely an aquatic plant towards which there is not a perpetual flow of the food- 
salts it requires in a form suitable for absorption. In connection with this kind of 
food-absorption there is also the fact that the parts by which hydrophytes attach 
themselves to a support are relatively small in area. Fucoids, as large as hazel 
trees in height and girth, are fixed to submerged rocks by groups of cells perhaps 
only 1 cm. in diameter. 
The quantity of food-salts absorbed by hydrophytes is very considerable com- 
pared with the amounts absorbed by other plants. As has been mentioned before, 
soda and iodine play a very important part in the thousands of different varieties 
which live in the sea. If Floridez are transferred from the sea into pure distilled 
water, common salt and other saline compounds diffuse out of the interior of the 
cells through the cell-membranes into the fresh water around. The red colouring 
matter of these Floridez also passes through the cell-walls into the water, proving 
that the molecular structure of the membrane is adapted to the agency of salt 
water in the osmotic processes of food-absorption. 
Plants living in fresh, or in brackish water, likewise absorb relatively large 
quantities of food-salts; and this accounts for the fact that water which is very 
poorly provided with nutriment of the kind contains only very few vegetable 
species. 
One would expect that exceedingly abundant vegetation would be evolved in 
running water, provided the latter contained food-salts in solution, however small 
they might be in quantity. For, in such a situation, it is not necessary to wait for 
the salts withdrawn by the plants from their immediate environment to be restored 
by the slow processes of mixture and equilibration; the water which has been drained 
of nutriment is replaced the next moment by other water bearing fresh food-salts. 
Experience shows, however, that flowing water is not so favourable to the develop- 
ment of hydrophytes as is the still water of pools, ponds, and lakes. This may 
partly depend on the fact that running water is always poorer in food-salts, and 
