160 Mr. W. Murtou Holmes o« the 



cells. It is supposed that when salts are removed by suction a 

 restitution of like salts immediately takes place from the sur- 

 rounding mould, so that the concentration of the solution re- 

 tained by the earth remains approximately the same, or at any 

 rate equilibrium is very quickly restored. One advantage of 

 this is that cells in immediate contact only meet with a saline 

 solution of constant weak concentration, and are secure from in- 

 jury which would result from contact with a very strong 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 a 

 degree of strength best suited to the plants concerned. 



Naturally the passage of salts from the earth to the interior of 

 the plant is dependent on the aid of water containing both the 

 substances composing cell-contents and the food-salts in solution. 

 The aqueous films adhering to the particles of earth, the water 

 saturating the cell-membrane, and the liquid inside the cell are 

 in unbroken connection, and along this continuous waterway the 

 passage of salt-molecules in and out can take place easily. The 

 majority of land-plants have special absorptive cells embedded 

 amongst or lodged upon the earth-particles, and are usually in 

 intimate connection with portions of them. Any part of the 

 plant that penetrates into the earth, or lies upon it, may, if it 

 performs the function of absorption, be equipped with cells of 

 this kind, but they are most commonly found close behind the 

 growing tips of roots ; they consist of delicate tubes resembling 

 fine hairs, and have received the name of root-hairs. These 

 root-hairs, as in the case of most seedlings and in grasses, are 

 generally thickly covered with particles of earth. In addition to 

 absorption of nutritive salts by root-hairs, there is also in many 

 cases an interchange of materials ; that is to say, not only do 

 substances infiltrate from the earth into the absorption-cells, 

 and so onwards into the tissues of the plant, but others pass 

 out of the plant through the absorption-cells into the earth. 

 Amongst these substances carbonic acid in particular plays an 

 important part, decomposing the earth-particles, and rendering 

 food-salts in immediate proximity available. 



Saprophytes are plants which take up organic compounds 

 formed during the process of decay. Among these are moulds 

 and other fungi, the mycelia, or vegetating portions, of which 

 often cover large areas, as in damp cellars, tunnels, mines, &c. 

 These plants contain no chlorophyll, and are in consequence 

 unable to form those compounds known as carbohydrates. 



Some plants, such as liverworts, ferns, and lycopods, although 

 containing chlorophyll, must be regarded as saprophytes ; so 

 that the presence or absence of chlorophyll is by no means a 

 distinguishing mark. 



