ABSORPTION OF FOOD, AND EXCRETION. 493 



and in the Monocotyledons silica, and in Dicotyledons lime and mag- 

 nesia, appear to be characteristic.* 



II. On the Absorption of Food and Excretion. 



194. We have now to take into consideration, without regard 

 to their intention, all those processes which occur on the external 

 surfaces of plants, by which they take up matter within their 

 structure, or by which they throw it off. 



1 . Of the Form of the Matter. 



All matters which plants either take up or throw off, must pass 

 through a homogeneous membrane moist with fluid, the cellular 

 membrane, and must also be soluble in water, as the only univer- 

 sal solvent. Only as fluids, vapours, or gases can they pass into 

 or out of the plant. Insoluble matters can never become the 

 food of plants by undergoing a chemical decomposition outside the 

 plant. 



Plants have no stomachs, nor the analogue of a stomach, and conse- 

 quently they have no digestion. The animal kingdom has a stomach, 

 in order to enable it to convert the nourishment received from a solid to 

 a fluid, from an insoluble to a soluble form ; then follows the absorption 

 of the nutriment through homogeneous membranes. But plants must 

 find all the substances requisite to their nutrition already in state of 

 solution ; they have no gastric juice by means of which they may chemi- 

 cally decompose and dissolve substances not ready prepared ; nor have 

 they salivary glands, in order to maintain the supply of a solvent juice. 

 The organic elements, carbon and nitrogen, are only present as carbonic 

 acid and carbonate of ammonia dissolved in water. Hence vegetation 

 is absolutely dependant upon water as a common solvent. Countries 

 that are entirely destitute of water are incapable of sustaining vege- 

 tation, as is the case with Sahara, a portion of the Gobiwiiste, c. ; 

 whilst the purest sand, if supplied with water, becomes capable of sup- 

 porting a vegetation, though it may be of a very poor and unproductive 

 order. Upon the supply of rain from the equator to the poles, and more 

 especially upon the supply of vapour in the atmosphere, the luxuriance 

 of vegetation is strictly dependant. 



The inorganic elements, as they are originally found in the firm 

 crust of our planet, are seldom or never soluble. Before they can be 

 used, a chemical process, aided by a mechanical one, must take place ; in 

 a word, they must be acted upon by the weather, in order that plants 

 may digest them. From these facts, two opposite considerations arise. 

 The matters which are taken up by plants must be soluble, but they 

 require to be not easily dissolved'!', or else they must be very gradually 

 set free from the insoluble matter with which they are in combination, 

 and should be yielded in very small quantities, in a soluble form. Plants 



* See Appendix A., 



f Upon this depends the secret of Liebig's patent manure. He maintains that he has 

 succeeded in replacing the easily soluble salts (matters) in the form of difficultly 

 soluble combinations. 



