HABITS OF PLANTS. 291 



In many cases perennial plants, by this change of climate, 

 are converted into annual ones ; that is, as if fearing the in. 

 clemencies of a cold winter, they pass through their successive 

 stages of existence with rapidity, and accomplish in one summer 

 what they had been accustomed to require years to perform. 

 The nasturtion was originally a shrub, flourishing without cul- 

 tivation on the banks of the Peruvian streams ; yet transferred 

 to this country it is an annual plant, which completes its term 

 of existence in a few months. 



The habits of some plants are with difficulty subdued ; and 

 it is by slow removals that they can be made to grow in foreign 

 situations. Rice by a slow progress has advanced from Caro- 

 lina to Virginia, and it is now cultivated in New Jersey. The 

 habits of Indian corn, aided by climate and culture, have suf- 

 fered a still more remarkable change. After having been for 

 several years raised in Canada, it arrives to perfection in a 

 few weeks, and on that account is employed by us as an early 

 corn ; but that which has been long cultivated in Virginia, will 

 not ripen in a New England summer ; yet, originally, the early 

 corn of Canada and that of Virginia were the same, both in 

 habit and other properties. 



Agents which affect the growth of plants. 



Of the various substances by which vegetables are nourish- 

 ed, water is thought the most important. Some plants grow 

 and mature, with their roots immersed in water, without any 

 soil ; most of the marine plants are of this description. 



Atmospheric air is necessary to the health and vigour of 

 plants ; if a plant is placed under a glass into which no air can 

 enter, it withers and dies. 



Most plants are found by analysis to contain a certain por- 

 tion of salts, such as nitre, and muriate of soda,* or common 

 salt. It appears that the root absorbs them from the soil, by 

 which it is nourished. 



No plants can grow without some degree of heat, though 

 some require a greater portion of it than others. 



Plants may be made to grow without light, but they will not 

 exhibit the verdure, or any of the properties of health. The 

 atmosphere, which is contaminated by the respiration of ani- 

 mals, is restored to purity by the vegetation of plants ; but se- 

 cluded from light, vegetables are no longer capable of convert- 

 ing a portion of the fixed air to their use, or of supplying the 

 atmosphere with the oxygen, on which its importance in sup- 



* According to modern chemistry, chloride of sodium. 



Some plants change from perennial to annual Of rice and Indian corn 

 A gents which affect the growth of plants Water Atmospheric air Salts 

 Heat Light. 



