II NUTRITION 3t 



stances of gradually increasing complexity, protoplasm, as 

 before, being the final result. 



To express the matter in another way : Amoeba can only 

 make protoplasm out of proteids already formed by some 

 other organism : Hsematococcus can form it out of simple 

 liquid and gaseous inorganic materials. 



Speaking generally, it may be said that these two methods 

 of nutrition are respectively characteristic of the two great 

 groups of living things. Animals require solid food con- 

 taining ready-made proteids, and cannot build up their pro- 

 toplasm out of simpler compounds. Green plants, i.e.., all 

 the ordinary trees, shrubs, weeds, &c., take only liquid and 

 gaseous food, and build up their protoplasm out of carbon 

 dioxide, water, and mineral salts. The first of these methods 

 of nutrition is conveniently distinguished as holozoic, or 

 wholly-animal, the second as holophytic, or wholly-vegetal. 



It is important to note that only those plants or parts of 

 plants in which chlorophyll is present are capable of holo- 

 phytic nutrition. Whatever may be the precise way in which 

 the process is effected, it is certain that the decomposition 

 of carbon dioxide which characterizes this form of nutrition 

 is a function of chlorophyll, or to speak more accurately, of 

 chromatophores, since there is reason for thinking that 

 it is the protoplasm of these bodies and not the actual green 

 pigment which is the active agent in the process. 



Moreover, it must not be forgotten that the decomposition 

 of carbon dioxide is carried on only during daylight, so that 

 organisms in which holophytic nutrition obtains are depend- 

 ent upon the sun for their very existence. While Amoeba 

 derives its energy from the breaking down of the proteids 

 in its food (see p. 15), the food of Hsematococcus is too 

 simple to serve as a source of energy, and it is only by the 

 help of sunlight that the work of constructive metabolism 



