164 SELECTIONS FROM ADDRESSES. 



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

 ble world is derived. But there is one principle of vegetable 

 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 nou- 

 rishment from one source, they are able to derive it from an- 

 other. It is well known that some plants will flourish in pure 

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

 destitute 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 ascertaining 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 conceal- 

 ed, yet we can see what are the most important agents con- 

 cerned ; 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 products of plants are the re- 

 sult of a similar galvanic agency exerted by their organs. The 

 necessity of heat for effecting these various changes has al- 

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

 •of light, and its mode of action, were understood. 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 chlorophylls. The green matter 

 itself is first produced, and then it forms an apparatus by which 

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



