164 AMEEICAN POMOLOGY. 



gredients of soil, of light, of heat, air, and moisturei 



2d— Those produced by poisonous agencies, as by in- 

 jurious gases, miasmata in the air, or by poisons in the 

 soil 



3d — Those arising from the growth of parasitic plants, 

 such as the various Fungi, Dodder, Mistletoe, etc. 



4th — Such as are caused by mechanical injuries or 

 wounds, and by the attacks of insects. 



These may be considered separately : 1st — ^It may be 

 assumed, and has been already well established by botan- 

 ists, that every plant has its own peculiar constitution, 

 adapting it to certain atmospheric conditions, and that for 

 its healthful and successful culture, these must be under- 

 stood and adhered to, within comparatively narrow limits. 

 Tropical plants, as is well known, cannot be cultivated 

 beyond their natural limits, except under circumstaiicea 

 where their natural conditions are nearly imitated by the 

 gardener ; and even in our stoves and hot-houses, these 

 plants do not compare in vigor with their fellows that lux- 

 uriate in the hot and steaming atmosphere of the tropics, 

 under the stronger light of such a clime as is natural to 

 them. On the contrary, the plants of northern latitudes 

 wiU not grow and produce seeds where temperature is too 

 elevated. Those from a humid atmosphere suffer in an 

 arid clime, and those which thrive in dry sandy regions 

 suffer equally when introduced into a humid atmosphere. 



Thus we find, that where there is too much moisture for 

 some of our cultivated plants, they are inclined to be to» 

 succulent, and this very excess may produce a dropsical 

 condition that is really a state of disease. Thus we suf- 

 fer in a loss of fruit, which will fall badly before its pe« 



