CARBOHYDRATES AND FATS 471 



although to attain these ends marked metamorphosis is not always 

 necessary (Sects. 22, 78, and icS). Indeed glucose, cane-sugar, oil, &c. 

 seem to function either as reserve or as translocatory material, whereas 

 starch and cellulose are capable of being translocated only when converted 

 into a soluble form ; inulin also seems to be used solely as reserve- 

 material. Metamorphoses of this character frequently involve a loss both 

 of energy and of substance, but it is easy to see, for example, that the 

 repeated solution and regeneration of starch (Sect. 108) during its passage 

 through chains of cells may be of considerable advantage, and this is 

 a conversion which is repeatedly performed when twigs are alternately 

 subjected to warmth and cold (Sect. 92). Only by considering the whole 

 of the aims and requirements of the plant can a correct estimation of any 

 phenomenon be obtained, and thus the formation of insoluble substances, 

 or of soluble compounds with greater molecular weights, such as inulin and 

 certain polysaccharides, not only renders exosmosis more difficult, but also 

 enables an excessive osmotic pressure to be avoided. Similarly, it is 

 apparently advantageous that seeds which are able to withstand desicca- 

 tion should contain mainly starch and oil in addition to proteids, soluble 

 and crystallizable compounds being avoided as far as possible. 



It is impossible to predicate on empirical grounds that a particular 

 substance must always be formed during translocation in order to render 

 a given metabolic process possible, for although reducing sugars, such as 

 glucose, commonly appear, and perhaps are also present even when in- 

 capable of detection, it is probable that a variety of different hexoses are 

 formed. Moreover, it has already been mentioned that similar products 

 may be formed from different sources, although the similarity becomes 

 evident only at a late stage in metabolism (Sects. 66 and 77). 



A final decision of these and similar questions will be possible only 

 when a more complete knowledge has been obtained of the inherent factors 

 concerned in metabolic activity. Mobilization and translocation are, how- 

 ever, simply preparatory partial functions, and hence are more amenable to 

 empirical study, as for example when some special decomposition is found 

 to be due to the action of an enzyme. The protoplast probably dissolves 

 the reserve-cellulose which encloses it by the aid of a ferment, and the hydro- 

 lytic decomposition of starch, inulin, cane-sugar, and other polysaccharides, 

 and possibly of fats also, is apparently produced in the same manner 

 (cf. Sect. 91). Enzymes are the agents employed by the protoplast when 

 extracellular action is necessary, and they are indispensable in preparing 

 substances for further change. There is no doubt, however, that an 

 organism which can produce amides and carry out complicated syntheses 

 will also be able to complete the simple changes of hydrolytic decomposi- 

 tion without extraneous aid. During translocation starch and various 

 polysaccharides may be repeatedly reconstructed and decomposed, and it 



