20 VITAL ACTIONS. 



when the peculiar principles, whether earthy or 

 saline, on which they naturally feed, are presented to 

 them, that they become perfectly healthy ; and espe- 

 cially when they have the means of obtaining nitro- 

 gen, which appears, from its great abundance in the 

 youngest parts, to be indispensable to plants upon 

 the first formation of their tissue.* 



39. In addition to their feeding properties,! roots 

 are the organs by which plants rid themselves of the 

 secreted matter which is either superfluous or delete- 

 rious to them. If you place a plant of Succory 

 in water, it will be found that the roots will, by 

 degrees, render the water bitter, as if opium had been 

 mixed with it ; a Spurge will render it acrid ; and a 

 leguminous plant mucilaginous. And, if you poison 

 one half of the roots of any plant, the other half will 

 throw the poison off again from the system. Hence 

 it follows, that, if roots are so circumstanced that 

 they cannot constantly advance into fresh soiL they 

 will, by degrees, be surrounded by their own excre- 

 mentitious secretions. 



40. It would also seem to follow that, under the 



* Mr. Riggs states that those seeds of the same kind, which con- 

 tain the largest quantity of nitrogen, germinate the earliest He 

 found nitrogen in young roots having the proportion of one to five 

 of carbon. Theodore de Saussure also ascertained that germinating 

 Seeds absorb this gas. 



f According to Mr. Knight, the roots of trees retain the original 

 vigour of the variety, after the trunks have become debilitated; or, 

 to use his own words, the powers of life do not become expended 

 so soon in roots as in bearing branches. (Set Rort. Trans., vol 

 u. p. 252.) 



