2 ANIMAL STUDIES 



distinguish all of them from all plants, or so to define 

 plants as to distinguish all of them from all animals. 

 While most animals have the power of locomotion, some, 

 like the sponges and polyps and barnacles and numerous 

 parasites, are fixed. While most plants are fixed, some of 

 the low aquatic forms have the power of spontaneous loco- 

 motion, and all plants have some power of motion, as espe- 

 cially exemplified in the revolution of the apex of the 

 growing stem and root, and the spiral twisting of tendrils, 

 and in the sudden closing of the leaves of the sensitive 

 plant when touched. Among the green or chlorophyll- 

 bearing plants the food consists chiefly of inorganic sub- 

 stances, especially of carbon which is taken from the car- 

 bonic-acid gas in the atmosphere, and of water. But some 

 green-leaved plants feed also in part on organic food. 

 Such are the pitcher-plants and sun-dews, and Venus-fly- 

 traps, which catch insects and use them for food nutrition. 

 But there are many plants, the fungi, which are not green 

 that is, which do not possess chlorophyll, the substance 

 on which seems to depend the power to make organic 

 matter out of inorganic substances. These plants feed on 

 organic matter as animals do. The cells of plants (in their 

 young stages, at least) have a wall composed of a peculiar 

 carbohydrate substance called cellulose, and this cellulose 

 was for a long time believed not to occur in the body of 

 animals. But now it is known that certain sea-squirts 

 (Tunicata) possess cellulose. It is impossible to find any 

 set of characteristics, or even any one characteristic, which 

 is possessed only by plants or only by animals. But nearly 

 all of the many- celled plants and animals may be easily 

 distinguished by their general characteristics. The power 

 of breaking up carbonic-acid gas into carbon and oxygen 

 and assimilating the carbon thus obtained, the presence of 

 chlorophyll, and the cell walls formed of cellulose, are char- 

 acteristics constant in all typical plants. In addition, the 

 fixed life of plants, and their general use of inorganic sub- 



