General physiology; an outline of the science of life . ussure, and, after havingbeen doubted for a long time, now forms one ofthe most important facts in all plant physiology. The plant snitrogen, however, as an experiment analogous to the above shows,cannot be extracted from the air ; it is taken up solely from thenitrogenous salts of the water. It follows from these experiments that plants construct theirliving substance out of simple inorganic compounds, from thecarbonic acid in the air, which is taken up by the leaves, andfrom the water containing salts, which reaches the plant throughits
General physiology; an outline of the science of life . ussure, and, after havingbeen doubted for a long time, now forms one ofthe most important facts in all plant physiology. The plant snitrogen, however, as an experiment analogous to the above shows,cannot be extracted from the air ; it is taken up solely from thenitrogenous salts of the water. It follows from these experiments that plants construct theirliving substance out of simple inorganic compounds, from thecarbonic acid in the air, which is taken up by the leaves, andfrom the water containing salts, which reaches the plant throughits roots. In contrast to this, no animal is able to build its living-substance synthetically from simple inorganic compounds, evenwhen all the chemical elements of its body are contained in them :all animals without exception require organic material alreadyprepared. This contrast between animals and plants is very significant, forit expresses the important fact that the animal world cannotexist without the plant world. It is true that a great number of. Fig. 4:2.—Corn-plant growing in a cylin-der containing anutrient Nutrient solu-tion : S, grain ofcorn ; K, cork.(After Sachs.) 140 GENERAL PHYSIOLOGY animals exist, such as carnivora, which require only animal food-stuffs, especially flesh ; but, if the source of their food besought, it is always found ultimately in herbivora, and thelatter cannot live without plant-food. Thus, the carnivora dependultimately upon the existence of plants. Without plants allanimals would die, for plants alone are able to manufacture frominorganic substances the carbohydrate, the fat and the proteid thatanimals require for their existence. The old philosophy of nature,prevalent at the beginning of the present century, was, hence, notentirely incorrect when in this sense it termed the whole animalworld parasites of the plants. For a long time it was believed that this difference in thenutrition of animals and plants is an absolute one, t
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