CHEMISTRY IN RELATION T<^> HORTICULTURE. 



103 



business, the other largely for pleasure ; agriculture is pursued for profit, 

 horticulture is almost regardless of such considerations ; agriculture is 

 carried out on a large scale, horticulture on a relatively smaU one. 

 Again, the persons interested in the two industries differ — for, while 

 farming, with but few exceptions, is largely the work of these who possess 

 but limited means, and who have to make it w pay its way," horticulture, 

 on the other hand, is pursued under conditions happily free from such 

 limitations. Nor are the aims alike, for while horticulture is mainly con- 

 cerned with the production of flower and foliage, agriculture has chiefly to 

 do with the finished product, the seed or grain, the straw or hay, the bulb 

 or root, and the utilisation of the same as the food of man or beast. So 

 also does the appearance of the area occupied differ, for, while in agri- 

 culture a rotation in cropping is largely foHowed, in horticulture the same 

 land is being continually devoted to the same plant. 8uch considerations 

 as these must largely affect the application of the general principles 

 I shall have to deal with, and limit their significance when horticulture is 

 speciaUy concerned. 



With these general remarks I shall proceed to take up in succession 

 the consideration of the main factors that have to do with plant life, in 

 their application to horticulture especially. 



The Atmospheee. 



In horticulture, as in agriculture, we are concerned in the first place 

 with the essential constituents cf the atmosphere — the oxygen, nitrogen, 

 and carbonic acid, the latter playing the chief part, and providing, along 

 with water, the main portion of the plant's structure. Plants derive the 

 whole, or nearly the whole, of their carbon from the atmosphere, the car- 

 bonic acid of which is taken in by the leaves, and. entering the cells, is, in 

 the presence of sunlight, and probably through the agency of the chloro- 

 phyll corpuscles, decomposed, transferring its carbon to the plant, where 

 it is variously elaborated, the products being conveyed to the different 

 parts of the plant, and going to build up the several tissues and cell- 

 contents. It is in this way that the woody fibre of plants is formed, this 

 being in effect the thickened cell-wall. The large class of carbohydrates, 

 comprising mainly the starches and sugars, are derived from this source 

 too, as also the gummy matters, the fats, and the vegetable acids. With 

 the aid of nitrogen, the carbonic acid goes further to build up the amides 

 and alkaloids, and, with nitrogen and sulphur, the albuminoids. As 

 examples of the above may be mentioned the starch of the potato, the 

 sugar of the beetroot, mangold, and turnip, the mucilage of linseed, 

 the fat of oil-seeds, the malic acid of apples or citric acid of lemons, the 

 amides found in asparagus or other immature nitrogenous vegetables, 

 the alkaloids of the tobacco plant and poppy, and lastly, the albuminoids 

 of peas, beans, and other leguminous plants. 



When this decomposition of carbonic acid goes on and carbon is 

 deposited in the plant, the other constituent of the carbonic acid, viz. 

 oxygen, is given out to the air. providing in return the means of sustain- 

 ing animal life, and keeping up thus the balance between plant and 

 animal requirements whereby each shall thrive. It is only, however, in 



