ABSORPTION OF FOOD-SALTS BY LAND-PLANTS. 85 
The salts are to be regarded as forming an extremely delicate coating round 
minute particles of earth where they are forcibly retained. If a plant rooted in 
the earth is to take in these salts it has to overcome the force by which their 
molecules are detained. This is effected, however, by means of a very powerful 
attraction exerted by the protoplasts of the plant as they grow, carry on the work 
of construction, and use up material. What actually happens is an energetic suction 
by the cells that are in close contact with particles of earth. This suction depends, 
however, upon the chemical affinity between the substances in the interior of the 
cells and the salts adhering to the earth-particles, as well as upon the consumption 
of food-salts for the manufacture of organic compounds within the green cells. It 
is supposed that whenever salts are abstracted from soil-particles by suction, a 
restitution of like salts immediately takes place, particles still unresolved in the 
immediate neighbourhood being dissolved, and a fresh influx taking place from the 
environment. Consequently the concentration of the solution retained by the earth 
is always approximately the same, or, at any rate, equilibrium is very quickly 
restored. One advantage of this is that the cells in immediate contact with 
particles of earth, and their adherent liquid, can only meet with a saline solution of 
constant weak concentration, and are therefore secure from injury such as would 
result in the case of most plants, from contact with a very concentrated solution. 
In other words, the absorptive power of earth acts as a regulator of the process of 
absorption of food-salts by plants, and is the means of keeping the saline solution 
in the earth always at the degree of strength best suited to the plants concerned. 
Naturally, the passage of salts from the earth to the interior of a plant is 
dependent on the aid of water containing both the substances composing cell- 
contents and the food-salts in solution. The cell-membranes, through which 
absorption takes place, are saturated with this solution. The aqueous films adhering 
to the particles of earth, the water saturating the cell-membrane, and the liquid 
inside the cells are really in unbroken connection, and along this continuous water- 
way the passage of salt molecules in and out can take place easily. 
The absorption of food-salts directly from the earth by green cells occurs very 
rarely. The protonema of Polytrichwm, which spreads its threads over loamy earth 
and wraps it in a delicate green felt, and that of the famous Cavern Moss (Schis- 
tostega), whose long tubular lower cells penetrate the earth in the recesses of caves, 
do undoubtedly suck up their necessary food-salts by means of cells containing 
chlorophyll. A drawing of the latter is given in Plate L, fig. p. 
The majority of land-plants have, however, special absorptive cells for the 
taking-up of salts in solution. These cells are imbedded amongst or lodged upon 
the earth-particles, and are usually in intimate connection with portions of them. 
Any part of a plant that penetrates into the earth or lies upon it, may, if it performs 
the function of absorption, be equipped with cells of the kind. Plagiotheeium 
nekeroideum, a delicate moss belonging to the flora of Germany, and growing on 
earth under overhanging rocks, where it is not exposed to rain, and therefore cannot 
receive any food-salts through that agency, develops absorption-cells on the apices 
