176 EXPERIMENTS WITH PLANTS 



In plants each cell probably absorbs its oxygen 

 directly from the air, which penetrates all parts of 

 the plant: in insects a somewhat similar method is 

 found, in that branching tubes convey the air to all 

 the viscera ; while in animals the air is conveyed to 

 the lungs, absorbed by the moist membranes of the 

 lung tissue, whereupon the oxygen combines with the 

 red coloring matter (haemoglobin) of the blood, 

 changing it from dark to bright red. The haemo- 

 globin is carried to all parts of the body (principally 

 in the arteries), where it gives up its oxygen to the 

 various tissues and is thereby changed back to dark 

 red and returns to the lungs (principally through the 

 veins) for a fresh supply. 



While both plants and animals need all three kinds 

 of food, i. e., proteids, fats and carbohydrates (sugars 

 and starch), they require them in different propor- 

 tions. In dietetics proteids are spoken of as flesh- 

 or muscle -formers, while starch, sugars and fats 

 are said to furnish fuel. This is not by any means 

 true absolutely, but it is probably so in the main. 

 The animal needs much more nitrogen-containing food 

 (proteid) than the plant: the animal excretes large 

 quantities of nitrogen (in the urine and feces) , while 

 the plant excretes none. It is interesting, in this con- 

 nection, to compare the composition of the food con- 

 tained in seeds with that contained in eggs. While 

 the white of a hen's Q^g^ for example, is almost pure 



