SECTION 16.] PLANT FOOD AND ASSIMILATION. 145 



directly or at second hand, the carnivorous upon the herbivorous; and 

 vegetables live upon earth and air, immediately or at second hand. 



446. The Food of plants, then, primarily, is earth and air. This is 

 evident enough from the way in which they live. Many plants will flourish 

 in pure sand or powdered chalk, or on the bare face of a rock or wall, 

 watered merely with rain. And almost any plant may be made to grow 

 from the seed in moist sand, and increase its weight many times, even if it 

 will not come to perfection. Many naturally live suspended from the 

 brandies of trees high in the air, and nourished by it alone, never hav- 

 ing any connection with the soil ; and some which naturally grow on the 

 ground, like the Live-forever of the gardens, when pulled up by the roots 

 and hung in the air will often flourish the whole summer long. 



447. It is true that fast-growing plants, or those which produce much 

 vegetable matter in one season (especially in such concentrated form as 

 to be useful as food for man or the higher animals) will come to maturity 

 only in an enriched soil. But what is a rich soil ? One which contains 

 decomposing vegetable matter, or some decomposing animal matter; that 

 is, in either case, some decomposing organic matter formerly produced by 

 plants. Aided by this, grain-bearing and other important vegetables will 

 grow more rapidly and vigorously, and make a greater amount of nourish- 

 ing matter, than they could if left to do the whole work at once from the 

 beginning. So that in these cases also all the organic or organizable matter 

 was made by plants, and made out of earth and air. Far the larger and 

 most essential part was air and water. 



448. Two kinds of material are taken in and used by plants ; of which 

 the first, although more or less essential to perfect plant-growth, are in a 

 certain sense subsidiary, if not accidental, viz. : 



Earthy constituents, those which are left in the form of ashes when a leaf 

 or a stick of wood is burned in the open air. These consist of some potash 

 (or soda in a marine plant), some silex (the same as flint), and a little lime, 

 alumine, or magnesia, iron or manganese, sulphur, phosphorus, etc., some 

 or all of these in variable and usually minute proportions. They are such 

 materials as happen to be dissolved, in small quantity, in the water taken 

 up by the roots ; and when that is consumed by the plant, or flies off pure 

 (as it largely does) by exhalation, the earthy matter is left behind in the 

 cells, just as it is left incrustiug the sides of a teakettle b which much 

 hard water has been boiled. Naturally, therefore, there is more earthy 

 matter (i. e. more ashes) in the leaves than in any other part (sometimes 

 as much as seven per cent, when the wood contains only two per cent) ; 

 because it is through the leaves that most of the water escapes from the 

 plant. Some of this earthy matter incrusts the cell-walls, some goes to 

 form crystals or rhaphides, which abound in many plants (422), some 

 enters into certain special vegetable products, and some appears to be ne- 

 cessary to the well-being of the higher orders of plants, although forming 

 no necessary part of the proper vegetable structure. 

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