470 ORGANOLOGY. 



which we have accurately examined the contents of the cells of the roots, 

 to vegetate in the same mixture of salts, we may thus ascertain how far 

 the simple absorption, as to quality and quantity, is affected by the 

 mere mixture of albumen, gum, and sugar in the interior of the cell. 

 We have not such experiments to adduce, or at least very few ; and we 

 must confess that with respect to the nutrition of the plant we know 

 scarcely anything. Part of this subject belongs to morphology ; the 

 materials which remain we may arrange under the following heads: 



I. The nutrition of the plant in general. 

 II. The absorption and excretion of the nutritious matter. 



III. The assimilation of the nutritious matter. 



IV. The external conditions of absorption and assimilation. 

 V. The motion of the sap in plants. 



I. The Food of Plants in general. 



190. The four elements which are essential to the formation 

 of all organised substances, namely, carbon, hydrogen, oxygen, 

 and nitrogen, are found in continual circulation in nature. We 

 find them in union with organic substances in the vegetable world. 

 The animal kingdom is entirely dependant upon vegetation for 

 support, either mediately (as the carnivora) or immediately (as the 

 vegetable feeders). By means of the vital functions of animals 

 (as respiration and perspiration), and by the corrupting and putri- 

 fying of their excrements, as also by the death of animals and 

 plants, and, finally, by the process of burning, organic substances 

 are continually converted into water, carbonic acid, and carbonate 

 of ammonia, which, as purely inorganic combinations, are held by 

 the atmosphere. These are again exclusively appropriated by the 

 vegetable kingdom, and restored to the domain of organised 

 matter. 



In an inductive inquiry into natural objects, before all things we must 

 strive after the discovery of leading maxims, which should be securely 

 founded, and by which we may decide upon the admissibility of hypo- 

 theses, and through which alone science can be made free from fiction.* 

 The paragraph above is a leading maxim of this kind, concerning the 

 changes of matter which go on in the three kingdoms of nature. In 



* In the following remarks a number of authorities will be used, and, in order to 

 avoid further reference, I have once for all given them here : 



1 . Humboldt's Reisen, and Essai sur la nouvelle Espagne. 



2. Codazzi, Resumen de la Geographia de Venezuela. 



3. Darwin's Voyage round the World. 



4. Blasius, Reise im Europaischen Russland. 



5. Ure, Dictionary of Arts. 



6. Macculloch, Dictionary of Commerce. 



7. Liebig, Organic Chemistry, in its Relations to Agriculture and Physiology. 



8. Boussingault, Economie rurale. 



9. Loudon, Encyclopaedia of Agriculture. 



10. Essays on German Agriculture, by Block, Schwerz, Schweizer, Hlubeck, &c. 

 in the Handbuch fiir angehende Landwirthe, by J. v. K. Leipsic. 



11. Lastly, I have used many private communications on methods of culture from 

 travellers and natives of Germany. 



