174 GENERAL PHYSIOLOGY 



in the fact that plants take in carbonic acid and give out oxygen, 

 while animals, vice versa, take in oxygen and give out carbonic 

 acid. But later experiments have shown that, in reality, this 

 contrast d(jes not exist. It is true that animals inspire oxygen, 

 employ it for the combustion of living substance, and expire 

 carbonic acid as the product of such combustion. But plants 

 do the same. In them this fundamental vital phenomenon 

 of respiration is merely concealed by the consumption and the 

 splitting-up of carbonic acid ; the latter, however, has nothing to 

 do with respiration itself, but is preliminary to the construction 

 by the plant of the first organic substance out of inorganic 

 materials. If the metabolism of plants be examined at a time 

 when no starch- formation is going on, when no carbonic acid is 

 being split up, but when the life of the plant is being expressed 

 in other ways, as in the night or in darkness, it is found, by 

 gasometric experiments analogous to those above described, that 

 the plant consumes oxygen and expires carbonic acid like the 

 animal. In the plant, therefore, the process of respiration is not 

 to be confounded with the process of assimilation of starch : the 

 latter requires carbonic acid to be taken in and split up and 

 oxygen to be given out, and thus conceals the respiration which 

 is constantly taking place beside it. Respiration, i.e., the taking-in 

 of oxygen and the giving-out of carbonic acid, is a general metabolic 

 phenomenon. 



Among liquid excretions ivater occurs everywhere, and sub- 

 stances dissolved in water. Because of the small quantity of 

 these various excretions, in the present state of micro-chemical 

 reactions it is usually not possible to demonstrate them for the 

 individual cell ; hence they must be studied in the compound cell- 

 community. In the plant, water is excreted and evaporated 

 during transpiration through the so-called stomata of the leaves. 

 By the action of special guard-cells the stomata can be closed and 

 opened, and thus the output of water by the plant can be 

 regulated very delicately. In animals there are special glands, 

 the kidneys and sweat-glands, the cells of which excrete the 

 water, together with the joroducts of retrogressive proteid-meta- 

 morphosis, out of the body-liquids, and pass them to the outside. 



Most of the non-nitrogenous products of proteid-decomposition 

 are oxidised completely to carbonic acid and water, so that the 

 latter leave the body as the almost exclusive end-products. But 

 intermediate products also arise, which, excreted by certain cells, 

 have a different fate within the body. This is true especially of 

 lactic acid, which, among other things, is excreted by the muscle- 

 cells into the blood and can be found there, but does not leave 

 the body as such in the urine. That sarco-lactic acid or para-lactic 

 acid is derived from the decomposition of proteids, and not from 

 the ingested carbohydrates, is proved by the experiments of 



