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 particular plants last cultivated have withdrawn from the land under tillage.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 laud-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- 

 ject 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. nutans — and a few white-flowered 

 Ranunculi — Ranunculus aquatilis and R. hololeucus), exhibit a transition stage from 



