402 Prof. H. Marshall Ward. The Relations between 



and similar carbohydrates, which yield the fuel and energy — as 

 they do in ordinary combustion — nevertheless, we must not fall into 

 the error of supposing that so much sugar or starch in the protoplasm 

 is forthwith and simply oxidised to t carbon dioxide and water, nor 

 may we conclude that the process is one of simple and direct oxidation 

 at all * 



In the first instance, it is chiefly owing to the vagaries of the 

 oxygen-molecules in the living protoplasm, that the latter exercises 

 the processes of metabolism, the second group of functions we have to 

 consider. 



The metabolic processes which can be referred to the changes 

 brought about during respirationf result in two series of events. On 

 the one hand, compounds of various kinds pass out of the protoplasm 

 — the arena of metabolic activity — into other parts of the cell, and 

 especially into the cell-sap; and, on the other hand, bodies of com- 

 paratively simple constitution are brought from the cell-sap and 

 elsewhere into the arena of activity, and there worked up into more 

 complex bodies. It is impossible to separate these two sets of pro- 

 cesses ; but, if we abstract them mentally, for purposes of simplicity, 

 we may say that the following series of events important for our 

 present purposes are taking place. 



Carbohydrates, especially in the form of glucoses, are being taken 

 up into the protoplasm, and built up into the structure of its sub- 

 stance : here, owing to the attacks of the oxygen of respiration, 

 the structures into which they enter are more or less broken down — 

 as before said, not necessarily merely oxidised as such or directly — 

 and the complex into which they have temporarily entered becomes 

 decomposed, again to be built up anew by the aid of more carbo- 

 hydrates, and so on repeatedly. 



Among the temporary products of these destructive processes, in the 

 complex alternations of building up and breaking down here going* 

 on, we find certain nitrogenous compounds (amides and allied bodies) 

 like asparagin, leucin, glutamin, &c, playing important parts. The 

 evidence goes to show that so long as plenty of carbohydrates are at 

 the disposal of the protoplasm, these amide-bodies are again worked 



* For the older literature, see Pfeffer, ' Pflanzenphysiologie,' vol. 1, p. 353 ; 

 Sachs, ' Lectures on the Physiology of Plants,' pp. 395 —408 : and Vines, op. ext., p. 

 214. Then consult Palladin, in £ Berichte d. Deutsch. Botan. Gresellsch.,' 1886, p. 

 322 ; 1887, p. 325 ; ' Botan. Centralblatt, 5 vol. 33, 1888, p. 102 ; Pfeffer, " Beitrage 

 zur Kenntniss der Oxydationsvorgiinge in lebenden Zellen" (' Abhandlg. Math.-phys. 

 Classe Sachsischen Gresellsch. d. Wiss.,' vol. 15, ~No. 5, 1889, pp. 375 — 518, especially 

 480 — 500), where the more important special literature is quoted. 



t Strictly speaking, metabolism includes all the chemical changes in the proto- 

 plasm -which constitute it living substance ; it is a mere convention to speak of 

 different kinds of m ^tabolism, and to separate carbon- assimilation as a special func- 

 tion. 



