ELEMENTS OF VEGETABLE PHYSIOLOGY. 



FOOD OF VEGETABLES. 



The food necessary for the development of the plant is drawn from the soil by 

 the root, and is absorbed by means of the spongioles which terminate the root-fibres, 

 and which are composed of a renewable cellular tissue having no epidermis. 



The substances drawn from the soil are, carbonic acid, ammonia, and alkaline 

 and earthy salts dissolved in water. Carbonic acid comes : 1, from the rain, which 

 has dissolved it in passing through the atmosphere ; 2, from the slow decomposition 

 of humus or mould, the carbon of which combines with the oxygen of the air, 

 which the water holds in solution. The ammonia comes : 1, from rain during 

 storms, when, by the influence of electricity, it is formed from the nitrate of 

 ammonia ; 2, from the putrefaction of vegetable or animal matter, at the commence- 

 ment of which azote and hydrogen combine. This decomposition is aided by adding 

 chalk to cultivated soil ; for chalk, as Boussingault has proved, attacks insoluble 

 azotized matters, and favours the formation of ammonia. The alkaline and earthy 

 salts, and notably the sulphates, and phosphate of lime are derived from the soil ; 

 the sulphates are decomposed by the ammonia, which substitutes itself as their base, 

 and forms a sulphate of ammonia, which, being soluble in water, and containing 

 azote, hydrogen, sulphur, and oxygen, is eminently adapted for the nourishment of 

 plants. Phosphate of lime, which is insoluble in pure water, is soluble in water 

 containing either an ammoniacal salt or carbonic acid only, as is the case with rain. 

 The water which holds in solution these different inorganic substances is a colourless 

 liquid, which rises by the vessels into the root, stem, and leaves, fills the cells and 

 their interstices, in which, during life, are formed the organic matters which 

 are- to be deposited in the tissue of the vegetable, or to assist in its growth. 



The above-mentioned inorganic substances are all binary compounds, which 

 sometimes remain isolated, sometimes enter into combination with one another. But 

 the organized substances which are found in the plant are the results of more com- 

 plicated combinations ; we have already spoken of cellulose and sta/rch, allied to which 

 is a third substance named dextrine, which does not turn blue with iodine and which 

 is soluble in water ; its chemical composition is exactly the same as that of 

 cellulose and starch, which are ternary bodies, composed of carbon, together with 



