THE BUILDING-STONES OF THE ORGANIC WORLD 



63 



their food directly from the surrounding atmospheric air. Thanks 

 to the presence of a certain substance contained in the plant- 

 cells, the chlorophyll or plant-green, plants are enabled to absorb 

 the carbon dioxide of the air and, under the chemical influence of 

 the sunlight, split it up into carbon and oxygen. The oxygen 

 is then largely exhaled whilst the carbon 

 enters with the water introduced into the plant 

 from the earth through the roots into the first 

 visible organic compound, starch. Starch is 

 the first and the only product of assimilation 

 from which all other organic compounds of 

 the plant are formed by means of chemical 

 metamorphosis. From starch we obtain all 

 other carbohydrates, fats, and, finally, the 

 highly complex albumen. Animals, on the 

 other side, inhale oxygen and exhale carbonic 

 acid, so that between the two kingdoms a 

 never-ending exchange takes place. It seems, 

 therefore, as if the presence or absence of the 

 chlorophyll, and the kind of alimentation con- 

 ditioned by it, is a fundamental factor in 

 differentiating between plant and animal. 



But there is found in every ditch and pool 

 a minute relative of the magnificent sea- 

 anemone, the little fresh-water polyp Hydra 

 viridis. This delicate little organism is distinguished by its 

 beautiful green colour, which on examination proved to be due to 

 chlorophyll. When this discovery was first made it seemed to 

 destroy the most important factor in distinguishing between 

 animal and plant, until it was further discovered that these 

 chlorophyll-corpuscles are not produced by the Hydra and do 

 not belong to its body, but are independent unicellular plants 

 (Algae), which penetrate from the outside into the body of the 

 Hydra and grow and multiply. 



Whenever we observe in Nature such intimate living-together 

 of two different organisms our first thought is that we have 

 before us a case of parasitism in which one part lives at the 

 expense of the other. But in this case we are observing an 

 intimate alliance, a so-called symbiosis, into which Hydra and 



FIG. 17. 



FRESH -WATER POLYP 



(Hydra viridis) ON 



THE ROOTS OF DUCK- 

 WEED (Lemna). 



(From v. Hanstein, 

 " Naturgeschichte 

 des Tierreichs. ") 



