ELEMENTARY VITAL PHENOMENA 



139 



If the root of a grain of corn that has sprouted in water be 

 placed in a cylinder containing this nutrient solution, while the 

 upper parts project into the air (Fig. 42), the plant, when placed 

 in the light, grows well, develops into a large 

 stalk, flowers and produces seed with which the 

 experiment can be repeated. If the iron salt 

 be wanting in the nutrient solution, the plant 

 grows likewise for some time, but remains colour- 

 less, and microscopic examination of the leaves 

 shows that the chlorophyll is wanting in the 

 cells. Only after the addition of a trace of iron 

 sulphate do the leaves become green. 



As a glance at the contents shows, no carbon 

 is present in the nutrient solution. Since, how- 

 ever, under all circumstances the plant requires 

 carbon for building its organic substance, in its 

 growth it must have taken carbon from the air ; 

 hence it is necessary that the experiment be ar- 

 ranged so that the upper parts of the plant pro- 

 ject into the air. If the air be excluded by a 

 bell-jar, in a short time the plant dies. Carbon 

 is contained in the air only in the form of car- 

 bonic acid ; hence the plant must withdraw it 

 from this compound, and, in fact, it appears that, 

 when a certain quantity of carbonic acid is left 

 under the bell-jar, after a short time all is con- 

 sumed. This important fact, that the plant 

 supplies its need of carbon solely from the 

 carbonic acid of the air, was discovered by 

 Ingenhouss and de Saussure, and, after having 

 been doubted for a long time, now forms one of 

 the most important facts in all plant physiology. The plant's 

 nitrogen, however, as an experiment analogous to the above shows, 

 cannot be extracted from the air ; it is taken up solely from the 

 nitrogenous salts of the water. 



It follows from these experiments that plants construct their 

 living substance out of simple inorganic compounds, from the 

 carbonic acid in the air, which is taken up by the leaves, and 

 from the water containing salts, which reaches the plant through 

 its roots. In contrast to this, no animal is able to build its living 

 substance synthetically from simple inorganic compounds, even 

 when all the chemical elements of its body are contained in them ; 

 all animals without exception require organic material already 

 prepared. 



This, contrast between animals and plants is very significant, for 

 it expresses the important fact that the animal world cannot 

 exist without the plant world. It is true that a great number of 



FIG. 42. Corn-plant 

 growing in a cylin- 

 der containing a 

 nutrient solution. 

 N, Nutrient solu- 

 tion ; S, grain of 

 corn ; K, cork. 

 (After Sachs.) 



