THE AGENTS WHICH AFFECT PLANTS. 



11 



or potting, when it stimulates them more than their crip- 

 pled roots will bear. Dull vveather, is, therefore, best for 

 both potting and planting, and a little shade after either 

 process may often be beneficial. 



3. — Air and Oases. 



Air is as necessary to plants as to human beings, since 

 they both exhaust it of its health-producing influences, and 

 probably both vitiate it to some extent, as far as them- 

 selves at least are concerned. There can be no general 

 healthiness or robustness without fresh air. The roots 

 of plants require to be within reach of it, consequently, 

 v>^here they are tolerably near the surface, and in porous 

 soil, the plants are much more fertile. Yery deep soils, 

 which attract the roots away from the influence of air, 

 tend only to the production of leaves and branches. Air 

 fills the soil as well as the atmosphere, and exists in plants 

 in little cells, which appear provided expressly for it. 



It is said to be valuable as a mechanical agent, in 

 agitating the different parts of plants, and keeping them 

 healthy and hardy, in helping off their watery evapora- 

 tions, and in removing impurities. But it is most useful 

 in conveying gases to them, as a very considerable 

 quantity of the gaseous food of plants resides in the 

 atmosphere, and is communicated to them directly 

 through their pores, or through the soil to the roots; 

 besides being precipitated upon them, or forced into the 

 ground for them by rain, snow, &c. Oxygen and nitrogen, 

 the food of plants, are the chief constituents of the atmos- 

 phere. Carbon, also, which is essential to plants, is 

 derived both from the air and the soil. It exists most 

 abundantly where population is densest, and the various 

 processes of life most thickly carried on. Plants and 

 trees in large towns must therefore tend materially to 

 improve the air, by relieving it of its carbon. 



A close frame or hand-glass, where little or no fresh 

 air is admitted to dry up the juices/ and that which it 

 contains is kept slightly moist, is the best condition for 

 newly potted plants or cuttings. Quiet moist weather is 

 likewise always best for planting, as winds seriously dry 



