14 



" As regards the diet of plants ^nd animals respectively, we have 

 already seen that the former cannot take in solid materials. It is far 

 different with animals, the most humbly organised of which have th© 

 power in some way or another of introducing solid food into their 

 interior and of digesting it. The nutriment of animals differs, 

 therefore, from that of plants physically. Another difference consists 

 in its chemical nature. An animal not only feeds on solid food, but 

 that food is of organic nature ; in other words, the animal enjoys the 

 privilege of eating its fellow creatures, dead or alive. A plant is, 

 however, not confined absolutely to inorganic matter for its diet. It 

 thrives upon and indeed requires organic matter, or the products 

 of organic matters ; but they must, as a rule, be waste products, not 

 living. Plants manured with purely mineral ingredients not only do 

 not thrive, but they are often worse off than others of the same kind 

 that are not manured at all. It is pretty clear, then, that plants 

 cannot live solely upon inorganic materials. Our every day experience 

 shows us, on the other hand, that animals cannot live exclusively upon 

 organic materials. If we want to feed a plant so as to ensure the 

 greatest amount of vigour, we give it organic food in the shape of 

 manure. If we want to digest our own food we take a sufficiency of 

 salt ; we give chalk to our chickens, lime and iron to the weak and 

 delicate of our own species. 



"The gaseous food of plants has already been alluded to. It remains 

 to notice the breathing process in plants and animals respectively. 



" Respiration is an interchange of gases, and this interchange is 

 effected in animals by means of cavities, lungs, gills, or tracheae ; but 

 whatever shape the breathing apparatus assumes, the ultimate result is 

 that the air inspired or expired is passed through a membrane. We 

 know from the researches of Grraham that membranes act the part of 

 filters, allowing some gases to pass and retaining others, according to 

 the nature of the filter and of the gases. 



" In the lower animals, and in plants, we have no special lungs or 

 gills. There are trachese in plants, but they are not specially sub- 

 servient to respiration ; and there is covering the whole surface a 

 thin membranous cuticle or epidermis. This cuticle acts as a filter, 

 allowing the gases to pass by diffusion into or out of the leaf. In 

 addition, there is a direct passage of gases through the stomata. 



" In a general sense, then, the mechanism of respiration is the 

 same in animals and plants. The movement of the leaves by the wind 

 probably serve the same purpose as that fulfilled by the muscles of 

 respiration in the case of an animal. The gases exhaled and inhaled 

 during the breathing process are the same in both instances. Con- 

 stantly there is an absorption of oxygen and a disengagement of car- 

 bonic acid gas. The elimination of oxygen by the green parts of plants 

 is, as before stated, not so much an act of respiration as of digestion. 

 The plant is thus constantly exposed to two antagonistic forces — the 

 one tending to build up, the other to destroy, the organism. At cer- 

 tain epochs in certain organs, and if kept in darkness, the plant 

 invariably emits the same gases as an animal does. So also, under the 

 influence of green light, the ordinary action of leaves is partially 

 inverted. In alluding to the effect of light on the disengagement of 

 oxygen gas, it should, however, he remembered that the light can be 

 and is fixed or stored in the plant, so as to operate for a time even in 

 complete darkness." 



