RESPIRATION 191 



at the end of summer contain as much organic material as would be equivalent 

 to the sum of the amounts assimilated during the several days of the annual 

 period of metabolic activity. The difference between the total amount assimi- 

 lated and the total amount dissimilated is the increase in dry weight — the 

 net result of normal plant growth. Plants may be easily cultivated under 

 conditions where assimilation is prevented or greatly retarded (e. g. in the 

 case of autotrophic plants grown in darkness, or of heterotrophic plants in 

 absence of nutrients), and under such circumstances destructive metabolism 

 still goes on and continued growth now results in a diminution in dry weight. 



This is admirably shown by a study of seedlings which have been grown 

 in the dark, though at first sight such a conclusion is by no means obvious. 

 The seedlings grow from day to day, and roots and shoots increase markedly 

 in volume, but that increase is entirely due to absorption of water, and the dry 

 weight, and more especially the organic material, decreases daily. The following 

 summary of one of Boussingault's experiments shows this clearly (Detmer, 

 1880, p. 247) : — 



Material. 



46 wheat grains 

 10 peas 



Dry weight of seeds. 

 1.665 gr. 



Dry weight of seedlings 



several weeks old, grown in 



darkness. 



0.713 gr. 



2.237 gr. I 1.076 gr. 



Loss. 



0.952 gr. 

 i.i6igr. 



A comparison between assimilation and dissimilation is more readily made 

 in a fungus than in one of the higher plants, for we have only to determine how 

 much nutritive material (e. g. sugar) the fungus has absorbed, how much dry 

 substance it has formed from it, and how much it might have formed. As 

 a basis for this last calculation we reckon that a fungus can construct about 

 2 gr. of dry weight for every gram of cane sugar absorbed, instead of which we 

 find that only 0-4 gr., or 0-5 gr., or even less, is produced. Pfeffer (1895, 257) 

 and KuNSTMANN (1895) have termed the numerical relation between the sugar 

 used up and the fungal substance formed the ' economic coefficient '. Theo- 

 retically the minimum value of this coefficient is about i, but in reality it has 

 always been found to be greater than unity. Kunstmann (1885) gives it as 

 from 1-13 to 3-38, and Ono (1900) obtained a value as high as 6-i, so that we 

 must not consider the coefficient as in any sense a constant, nor look on the 

 plant as always an economical worker. The coefficient increases, for example, 

 with the progressive development of the fungus and with elevation of tempera- 

 ture. Among other external influences poisons must be specially noted, the 

 stimulating eSects of which in weak doses we have already drawn attention to. 

 Thus it has been shown by Ono that the addition of a 0-003 per cent, to a 0-03 

 per cent, solution of zinc sulphate reduced the economic coefficient, in the case 

 of Aspergillus, from 6 (or 4 in other experiments) to about 2-8. The chemical 

 stimulus resulting from the addition of such substances induces an economy 

 in the expenditure of food materials. The deficit appearing from day to day 

 under all conditions is the result of dissimilation. 



No organism can remain in existence without constantly losing weight 

 from the dissimilation or destruction of organic substance. We may term 

 this katabolic process respiration, whatever be the nature of the final pro- 

 ducts, or we may reserve this name for those destructive changes which are 

 exhibited by most plants under ordinary conditions and which result in the 

 formation of carbon-dioxide and water from organic materials. Of these 

 products of decomposition the most obvious one is carbon-dioxide ; the 

 j)roduction of water is much less easily demonstrated. Both carbon- 

 dioxide and water may be obtained by decomposition of organic materials 

 such as starch, sugar, &c., not merely in the course of respiration in the 

 organism but also by ordinary combustion. Hence we may conclude that 

 respiration is a combustion process, and every experiment goes to show 



