444 MANUAL OF BOTANY 



the accumulation of food material in various parts, to be there 

 held in reserve till the plant requires them. 



It is not certain that the process of construction of carbo- 

 hydrate is so simple as w^iat has been said would imply. There 

 are many other chemical compounds in the cell while it is pro- 

 ceeding, and to what extent they take part in the matter is not 

 known. It is held that some compound of potassium, at any 

 rate, is concerned in the process, for in the absence of this metal 

 sugar does not appear to be formed. 



The formation of the proteid l)odieS in the cells is not at all 

 well understood ; indeed, but little is known certainly about it. 

 The nitrates are absorbed and taken to tlie mesophyll cells. In 

 these cells and in others about the plant, bodies occur of com- 

 paratively simple composition, though much more complex 

 than nitrates. These, which are known as amides, include as- 

 paragin, leucin, and several others. Probably they are the first 

 bodies constructed from the nitrates, though in many cases they 

 arise from the decomposition of proteid and not from anabolic 

 processes. It is in the form of these bodies generall3' that 

 organic nitrogenous matter travels about the plant. From them 

 most likely proteid is constructed, compounds of sulphur and of 

 phosphorus in the cells supplying those elements, which, as we 

 have seen, are probably both in the molecule of proteid. Cer- 

 tainly sulphur enters into its composition. 



Again there is reason to think that the carboh^'drate manu- 

 facture is associated with the nitrogenous one. Proteid can 

 apparently only be formed under the same conditions as carbo- 

 hydrate, though we cannot say that chlorophyll is directly con- 

 cerned. If a shoot is not exposed to light, there will be an 

 accumulation of asparagin in its tissues ; the same thing will be 

 observed if CO., is excluded from access to it. Indeed, if carbon 

 assimilation is prevented, nitrogenous material cannot be formed. 



The proteids, if formed in the leaA^es, as is probably the case, 

 are affected just as we have seen the carbohydrates to be. They 

 undergo a partial decomposition, and their nitrogenous residues 

 leave the cells in the form of amides. Their further fate will be 

 examined later. 



The proteid, which is the most complex body known to exist 

 in the plant with the exception of the living substance itself, 

 cannot at present be represented by any chemical formula. 

 Analyses of the purest forms of it point to its percentage compo- 

 sition lying within the following limits : — 



