Nutrition of Plants. 159 



supply is being constantly worked up by the chlorophyll bodies. 

 In this way starch, sugar, and other similar organic compounds 

 are produced. 



In the case of plants living under water the supply of carbonic 

 acid is derived from the surrounding water, where it is held in 

 solution. It is also taken up in combination with lime as bi- 

 carbonate. Part of the carbonic acid is withdrawn, and the 

 reduced mouo-carbouate precipitated. Hence the incrustation 

 of plants in both fresh and salt water. 



Although nitrogen is present in the air to the extent of 79 per 

 cent, by volume, none is absorbed by plants in the free condition.* 

 It is derived almost entirely from the nitrates and ammoniacal 

 compounds formed in the soU from decomposing organic sub- 

 stances. 



Mineral salts can only pass through cell-membranes in a state 

 of solution ; hence soluble sulphates, phosphates, and chlorides 

 of calcium, magnesium, potassium, and iron may pre-eminently 

 be called food-salts. As the plants remove these from the soil 

 it is necessary to replace them from time to time by means of 

 manures of different kinds, and, as there is the power of selec- 

 tion, one plant preferring one substance, and another another, 

 farmers find it expedient to vary their crops in rotation, so that 

 the land may not become too much impoverished. Food-salts 

 are absorbed by water-plants from the surrounding water through 

 their whole surface, and their structure is simpler than is the 

 case with land-plants. 



In no class is absorption of mineral food-salts accomplished 

 in so complicated a manner as in land-plants, and the process is 

 by no means uniform in different plants. Every kind of earth, 

 especially that rich in clay and humus, which is a substance 

 produced by the decomposition of plants and animals, has the 

 power of retaining gases, and especially water and salts. 



Salts are to be regarded as forming an exceedingly delicate 

 coating round the minute particles of earth, where they are 

 forcibly retained. If a plant rooted in earth is to take in these 

 salts, it has to overcome the force by which these molecules are 

 detained. This is effected by the attraction exerted by the 

 protoplasts of the plant as they grow, carry on the work of con- 

 struction, and use up material. What actually happens is an 

 energetic suction by the cells that are in close contact with 

 particles of earth, depending upon the chemical affinity between 

 substances in the interior of the cells and the salts adhering to 

 earth-particles, as well as upon the consumption of food-salts 

 for the manufacture of organic compounds within the green 



' Certain bacteria forming colonies on the roots of leguminous plants 

 enable the latter to absorb free nitrogen. 



