ABSORPTION OF FOOD-SALTS BY WATER-PLANTS. 75 
the results of scientific research, have been introduced into practice. Amongst 
these processes may be mentioned the rotation of crops, the artificial application of 
manure to exhausted land, and the restitution of the mineral food-salts which the 
particular plants last cultivated have withdrawn from the land under tillage. 
ABSORPTION OF FOOD-SALTS BY WATER-PLANTS. 
It is usual to designate all plants that grow in water as hydrophytes or water- 
plants. But in their narrower sense these names are only applicable to those plants 
which, during their entire lives, vegetate under water and derive their nutriment, 
especially carbonic acid, direct from the water. A number of plants have widely 
ramifying roots fixed in the earth at the bottom of water, and the lower parts of 
their stems, either temporarily or throughout life, immersed in water, whilst the 
upper parts of their stems and their upper leaves are exposed to the air and take 
carbonic acid direct from the atmosphere, and these should be regarded as marsh- 
plants and classed with land-plants so far as regards food-absorption. Reeds and 
rushes, water-fennel and water-plantain, the yellow water-lily, even the amphibious 
Polygonum and the white water-lily, are marsh-plants and not true hydrophytes. 
It is characteristic of all these marsh-plants, that-if they are entirely submerged 
for any length of time they die, whereas they are not injured if the water's level 
at the place where they grow sinks so as to expose the lower portions of the stem. 
In places formerly submerged, but from which, in course of time, the water has 
retreated, so that they have been turned into meadows, one may come across not 
only clumps of reeds and rushes but even yellow and white water-lilies, flourishing 
perfectly on the moist earth. 
Water-plants, or hydrophytes in the proper acceptation of the term, perish 
if they are kept for a length of time out of their proper medium and exposed to 
the air. In most of them death ensues quickly, for their delicate cell-membranes 
are not able to prevent the exhalation of water from the interior of their cells; 
and, there being no provision for a replacement of the evaporated fluid, the 
whole plant dries up. If one supplies aquatic plants, thus desiccated, with 
water, though it is indeed absorbed it no longer has the power of reviving them. 
Those hydrophytes which occur in the sea, near the shore, are able to stand 
exposure to the air for a comparatively long time, and they are regularly sub- 
jeet to it during ebb-tide. Sea-wracks which at high-tide were floating in the 
water are then seen lying on the dry rocks or sand of the shore. But the mem- 
branes of the cells forming the outermost layer in all these sea-wracks is very thick. 
They retain water staunchly and prevent the plants from drying up, at least until 
high-tide occurs again, when they are once more submerged. 
Amphibious plants in which the lower leaves are like those of aquatics and the 
upper like those of land-plants so far as desiccation is concerned (e.g. several kinds 
of pond-weed—Potamogeton heterophyllus and P. natans—and a few white-flowered 
Ranuneculi— Ranunculus aquatilis and R. hololeucus), exhibit a transition stage from 
