Chap. 111.] VEGETAL ASSIMILATION AND DIS- ASSIMILATION. 115 



anything to the nutrition of plants ; they are mechanically driven 

 by the water which penetrates into the plant, and are deposited 

 in the deep tissues, after having formed insoluble salts with the 

 vegetal acids. 



Habitually, these mineral particles are uniformly intercalated 

 between the organic molecules ; also by incineration, or by 

 treating the vegetal anatomical elements with certain acids, we 

 may succeed in destroying the organic substances which they 

 contain, preserving only the mineral skeleton, but nevertheless 

 keeping the form of the destroyed anatomical element. 



Often the mineral salts also form true crystals in the vegetal 

 tissues and even in the cells. Such, for example, are fine granulous 

 incrustations of carbonate of lime, bundles of spars of oxalate of 

 lime, or even tolerably voluminous crystals. 



The amount of mineral matters in a plant is proportioned to 

 its age, and also to the quantity of water which flows through it ; 

 in short, to the activity of the vegetation. The proportion of 

 soluble substances may undergo certain variations. As to the 

 insoluble substances, they accumulate ceaselessly, mineralizing 

 the plant more and more, and contracting the range of action of 

 life. If most of the mineral substances drawn from the soil 

 are of little use in the development and nutrition of plants, yet 

 some of them are very important, for example, ammonia, the 

 phosphates and the sulphates. Others again appear sometimes 

 necessary to one plant or group of plants, sometimes to another. 



Most plants which grow on the seashore contain much soda, 

 and this soda is necessary to them ; for they grow only upon the 

 seanjoast or near saline deposits inland. 



We always find in the tissues of plants of certain very natural 

 families, the same mineral substances. They have thus chosen 

 them in a certain degree. We may mention the gramineous 

 plants, the stems of which, without exception, contain silica, 

 whilst the fruits contain phosphate of magnesia and of ammonia. 



But often the same plants contain different salts, according as 

 they grow in various- soils. 



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