50 MASSACHUSETTS AGRICULTURE. 



atmosphere in the condition of common salt, which he has 

 shown to exist probably in the air. 



Such are the ordinary sources whence the food of the vege- 

 table world is derived. But there is one principle of ve'getable 

 physiology of great importance and deep philosophical interest, 

 that should be noticed, since its neglect has been a fruitful 

 source of controversy among scientific men. It cannot be 

 doubted that plants, like animals, have the power of adapting 

 themselves to circumstances ; so that, if they cannot obtain 

 nourishment from one source, they are able to derive it from 

 another. It is well known that some plants will flourish in 

 pure water, others suspended in the air ; and on some marly 

 soils, destitiite of vegetable matter, abundant crops may be 

 raised without manure through an indefinite number of years ; 

 though in all these cases, probably, plants are less robust and 

 prolific than if supplied with food both from the soil and the 

 air. But their ability to draw a less perfect subsistence from 

 different sources is another interesting evidence of Divine fore- 

 sight and benevolence. 



Chemists have not been satisfied with ascertaininci: the nature 

 and origin of the nutriment of vegetable nature. They have 

 attempted to follow the crude materials through their various 

 and most delicate metamorphoses, till they become converted 

 into the different remarkable compounds which plants produce. 

 And though much of the chemistry of these changes is con 

 cealed, yet we can see what are the most important agents 

 concerned ; and heat, light, and electricity, stand at the head of 

 the list. It can hardly be doubted that the rootlets of plants 

 have the power, by a galvanic agency, of eliminating from the 

 soil many important principles, not otherwise separable ; nor is 

 it more doubtful that the various jDroducts of plants are the 

 result of a similar galvanic agency exerted by their organs. 

 The necessity of heat for effecting tliese various changes has 

 always been known ; but it is not till recently that the necessity 

 of light, and its mode of action, were mnjerstood. The sap, it 

 appears, undergoes but little change till it reaches the leaves. 

 There it experiences digestion, by the action of solar light upon 

 the green matter, called chlorophijlle. The green matter itself 

 is first produced, and then it forms an a})paratus by which the 

 compounds in the sap are decomposed, the oxygen gas is liber- 



