294 THE FOOD OF PLANTS 



relation to their respective weights, the respiratory activity is often much 

 greater in plants than it is in a mammal, although in the former no constant 

 body temperature has to be maintained (Sect. 95). The mere fact that so 

 many non-green organisms exist should suffice to show that the metabolism 

 in plants is not essentially different to that in animals, although this is an 

 error which is still frequently made l . 



No definite line of demarcation can be drawn between animals and 

 plants, either from a physiological or a morphological standpoint, for both 

 have a common origin, and it is only in the higher forms that highly 

 specialized structures and peculiar physiological properties have been 

 acquired. Hence it is naturally in the lower forms that the resemblances 

 and points of similarity are most marked, but all embryonic protoplasts are 

 similarly constructed and constituted. At the same time, in plants as well as 

 in animals, special substances may be formed, either for definite purposes or 

 as the unavoidable by-products of a slightly different metabolism. Cellulose 

 is a product of this nature, and is very largely employed by plants as 

 a mechanical and supporting framework ; in animals it occurs so rarely 

 that its presence was formerly erroneously supposed to indicate the 

 vegetable nature of an organism. There is indeed less difference between 

 the metabolic products of certain plants and animals than exists between 

 those of some species of bacteria, fungi, and other plants. Little is known 

 concerning the metabolism of the lowest animals, but it is probably very 

 similar to that of plants, and, as time goes on, more and more of the substances 

 supposed to be characteristic of animal metabolism are being discovered in 

 plants. It is possible that many of the lower animals can form proteids by 

 synthesis, while plants exist which have not this property, and which must 

 therefore obtain their proteid food from other plants or animals. How- 

 ever the proteids may be obtained, they are subjected to various changes in 

 the course of metabolism, and indeed with certain forms of nutriment 

 a fungus may be only able to grow by completely decomposing the 

 proteids supplied to it (cf. Sects. 77, 80, 64, 68). 



In the progress of their phylogenetic development most plants have 

 become adapted to a permanently non-motile existence, and are hence unable 

 to seek their food in the way that most animals do, although various 

 forms exist which occupy an intermediate position as regards their mode 

 of nutrition. Animals commonly swallow solid food, whereas plants always 

 absorb it in solution ; but this is by no means an essential point of 

 difference, for the solid food is digested and made soluble in the stomach 

 of an animal in a manner similar to that in which the pitcher of a carni- 

 vorous plant digests a piece of meat. Many fungi are also able to excrete 



1 Cf. Pfeffer, 1. c., p. 202. 



