5 io THE POPULAR SCIENCE MONTHLY. 



of plants known as proteids, starch, cellulose, etc., are formed in 

 the downward steps of its progress with a liberation of a portion 

 of the stored energy in the form of heat. The heat liberated in 

 these first steps of destructive metabolism is not, however, suffi- 

 cient to maintain an independent temperature in plants, as it is 

 used in vaporizing the water exhaled in the processes of growth, 

 or lost by radiation from the extended surface of foliage. The 

 energy expended in vaporizing water must be considerable, as ex- 

 periments show that for each pound of dry organic substance 

 formed by the plant about three hundred pounds of water are ex- 

 haled in the form of vapor.* 



The products of the downward steps of metabolism are numer- 

 ous, some of which, as waste matters, are either excreted, as in the 

 case of carbonic acid and water, or deposited in the more stable 

 tissues ; while others called plastic products, including proteids, 

 starch, fats, etc., are stored as reserve materials to be used in con- 

 structive metabolism when needed by the plant. These reserve 

 materials are not as a general rule stored in the place where they 

 are formed, and they can only be transported when changed to a 

 soluble form, which is brought about by certain " soluble fer- 

 ments/' which are also products of the destructive metabolism of 

 protoplasm. Starch formed in the leaves is changed to glucose 

 and transported to other parts of the plants where it is recon- 

 verted into starch and stored for use, sooner or later, in construc- 

 tive metabolism. Some of these reserve materials are apparently 

 built up again into protoplasm before they are resolved into their 

 ultimate products. This is seen in the starch deposited in oily 

 seeds which is used in forming protoplasm, and the oil is then 

 formed as a product of its metabolism. 



The many forms of organic acids — as the malic, tartaric, oxalic, 

 citric, etc. — and a variety of alkaloids and other bodies are also 

 products of destructive metabolism, which may be deposited in 

 the various tissues as waste materials, or some of them may be 

 changed by soluble ferments into forms which may be again util- 

 ized in the economy of the plant. The organic acids and tannin 

 of green fruits, for example, are converted into sugar in the pro- 

 cess of ripening by ferments formed from the protoplasm of the 

 fruit cells. 



In general terms the processes of nutrition in plants may, then, 

 be said to consist in the construction of protoplasm from the ele- 

 ments of their food, with a storing of energy, and the conversion 

 of this protoplasm into the various organic substances entering 

 into the composition of their tissues (starch, cellulose, etc.), with 

 a liberation of energy, and all vital activities are included in 



* Popular Science Monthly, May, 1892, p. 92. 



