416 CORRELATION OF PHYSICAL AND VITAL FORCES. 



tions of growth and development, are dependent on the agency 

 of Heat, is just as evident in the stage of maturity as in that 

 of germination. And there is reason to believe, further, that 

 an additional source of organizing force is to be found in the 

 retrograde metamorphosis of organic compounds that goes on 

 during the whole life of the plant ; of which metamorphosis 

 the expression is furnished by the production of carbonic acid. 

 This is peculiarly remarkable in the case of the Fungi, which, 

 being incapable of forming new compounds under the influ- 

 ence of light, are entirely supported by the organic matters 

 they absord, and which in this respect correspond on the one 

 hand with the germinating embryo, and on the other with 

 Animals. Such a decomposition of a portion of the absorbed 

 material is the only conceivable source of the large quantity 

 of carbonic acid they are constantly giving out ; and it would 

 not seem unlikely that the force supplied by this retrograde 

 metamorphosis of the superfluous components of their food, 

 which fall down (so to speak) from the elevated plane of " prox- 

 imate principles," to the lower level of comparatively simple 

 binary compounds, supplies a force which raises another por- 

 tion to the rank of living tissue, thus accounting in some de- 

 gree for the very rapid growth for which this tribe of Plants 

 is so remarkable. This exhalation of carbonic acid, however, 

 is not peculiar to Fungi and germinating embryos, for it takes 

 place during the whole life of flowering plants, both by day 

 and by night, in sunshine and in shade, and from their green 

 as well as from their dark surfaces ; and it is not improbable 

 that, as in the case of the Fungi, its source lies partly in the 

 organic matters absolved, recent investigations * having ren- 

 dered it probable that Plants really take up and assimilate 

 soluble humus, which, being a more highly carbonized sub- 

 stance than starch, dextrine, or cellulose, can only be con- 



* See the Memoir of M. Jttsler, " On the Absorption of Humus," in 

 the "Bibliotheque UniverseUe," N. S., 1858, torn, i., p. 305. 



