-1859] WRITINGS OF JOSEPH HENRY. 121 



plant afterward we shall find that the amount of organic 

 matter contained in the latter is but a fraction of that which 

 was originally contained in the former. We can account in 

 this way for the disappearance of a part of the contents of 

 the sac, which has evidently formed the pabulum of the 

 young plant. But here we may stop to ask another ques- 

 tion: By what power was the young plant built up of the 

 molecules of starch? The answer would probably be, by the 

 exertion of the vital force; but we have endeavored to show 

 that vitality is a directing principle, and not a mechanical 

 power, the expenditure of which does work. The conclusion 

 to which we would arrive will probably now be anticipated. 

 The portion of the organic molecules of the starch, &c., of 

 the tuber, as yet unaccounted for, has run down into inor- 

 ganic matter, or has entered again into combination with 

 the oxygen of the air, and in this running down, and union 

 with the oxygen, has evolved the power necessary to the 

 organization of the new plant. 



If we examine the skin of a potato, we shall find it perfo- 

 rated by innumerable holes, through which the oxygen 

 penetrates into the interior to enter into combination with 

 the starch, (or in other words, to burn it by a slow combus- 

 tion,) and through which the carbonic acid and vapor of water 

 again find their way into the atmosphere. We see from this 

 view that the starch and nitrogenous materials, in which 

 the germs of plants are imbedded, have two functions to 

 fulfil; the one to supply the pabulum of the new plant, and 

 the other to furnish the power by which the transformation 

 is effected, the latter being as essential as the former. In 

 the erection of a house, the application of mechanical power 

 is required as much as a supply of ponderable materials. 



But to return to our first supposition. We have said (and 

 the assertion is in accordance with accurate observation) that 

 the plant would cease to increase in weight under the mere 

 influence of heat, however long continued, after the tuber 

 was exhausted. Some slight changes might indeed take 

 place; a small portion of pabulum might be absorbed from 

 the earth ; or one part of the plant might commence to decay, 



