THE METABOLISM OF PLANTS. 195 



in certain substances which are brought within the sphere 

 of its influence, though we can give no satisfactory account 

 of the mode in which such decomposition is effected, any 

 more than we can of the mode in which unorganised ferments 

 act, or of the mode in which the decomposition of protoplasm 

 itself is brought about. We may, for the sake of precision, 

 term this the fermentative action of protoplasm, and though 

 we usually associate fermentative action with the so-called 

 organised ferments, such as Yeast and Bacteria, it is by no 

 means peculiar to them, for, as will be shewn hereafter, it is 

 manifestly exhibited by all living plant-cells when placed 

 under appropriate conditions. 



The characteristic accompaniment of the destructive meta- 

 bolism of plants, as of all living organisms, is, under normal 

 conditions, that interchange of gases between the plant and 

 the surrounding atmosphere which has already been mention- 

 ed (Lect. V., p. 81) as Respii^atwn, and wWch^consists in- the 

 absorption of oxygen and the evolution of carbon dioxide, 

 an interchange which is precisely the converse of that which 

 accompanies the constructive metabolism of green plants. 

 Carbon dioxide is not, however, the. only respiratory excre- 

 tum, for de Saussure, and, more recently, Laskowsky have 

 shewn that watery vapour is also exhaled in the respiration 

 of plants. Respiration involves, therefore, a loss of weight to 

 the plant, inasmuch as the absorbed oxygen is given off in 

 the excreted carbon dioxide and water. 



The following illustration of the loss in weight which is involved by 

 respiration is taken from Boussingault. 



Forty-six grains of wheat were sown on May 5 : the seedlings were 

 reaped on June 25, having been in darkness all the time. 



At the commencement 



Total Dry 



of the expt. the grains Weight. C. H. O. N. Ash. 



consisted of . . . 1-665 0758 0*095 7i8 f 0-057 0*038 grms. 



At the end of the expt. 

 the seedlings con- 

 sisted of 0712 0293 0-043 ' 282 '57 0*038 grms. 



Loss during experiment 0-953 0-465 0-052 0-436 o'ooo o'ooo grms. 



13-2 



