192 



METABOLISM 



that oxygen is essential to the maintenance of respiration. Respiration may, 

 therefore, be also described as an oxidation process, standing in marked con- 

 trast to carbon assimilation, which we have learned to regard as a process 

 of reduction. 



It will be necessary for us to study, in the first instance, the methods of 

 demonstrating respiration so that we may appreciate to what extent respiration 

 occurs in the vegetable kingdom. As a proof of its occurrence we shall employ, 

 as a general rule, the excretion of carbon-dioxide, a gas which may be measured 

 quantitatively and quahtatively without any difficulty. Place, for example, 

 a handful of germinating seeds in a flask, closed by means of a rubber stopper 

 through which passes a glass tube, and keep the tube closed for a few hours ; 

 then open the tube under lime water — the resulting cloudiness in the fluid 

 demonstrates that a certain amount of carbon-dioxide was present in the flask. 

 In place of lime water caustic potash may be employed, as this substance readily 

 absorbs carbon-dioxide and replaces the gas in the flask by way of the glass 

 tube. Perhaps the simplest, although a more indirect, method of all for 

 demonstrating respiration is based on the fact that oxygen is used up in pro- 

 portion to the carbon-dioxide given off. If we place some germinating seeds, 

 young leaves, buds, &c., at the bottom of a tall glass cylinder closed by 

 a stopper, and if after several hours we insert a burning taper into the jar 

 after careful removal of the stopper, we shall see from its immediate extinction 

 that the enclosed air has been deprived of most of its oxygen through the 

 activity of the vegetable structures within. 



It is obvious that both the method by absorption of carbon-dioxide by 

 caustic potash and that by precipitation of carbonate of lime on the addition of 

 lime water furnish us with a means of determining quantitatively how much 

 carbon-dioxide is produced during respiration, still the use of completely closed 

 spaces in such experiments is to be avoided, since respiration itself, under such 

 conditions, becomes abnormal owing to the rapid decrease in the amount of 

 oxygen present. It is preferable, therefore, to place the plant experimented 

 on in a vessel through which a continuous stream of air may be driven. This 

 air is deprived of all its carbon-dioxide before entering the vessel and becomes 

 once more charged with that gas within it ; the amount added may be easily 

 determined as the gas leaves the vessel at the other end. Into the purely 

 chemical details of the experimental method it is unnecessary for us to enter. 

 The amount of oxygen used up may also be employed as a measure of the amount 

 of respiration taking place, as well as the amount of carbon-dioxide produced. 



The first conclusion we arrive at from a study of comparative estimates 

 of the intensity of respiration is that different plants, different members of the 

 same species, and even the same organ of an individual plant in different stages 

 of development exhibit the widest possible variations. Certain biological groups, 

 such as oily and shade-loving plants, are remarkable for the feebleness of their 

 respiration, while on the other hand, many Fungi exceed the warm-blooded 

 animals in respiratory activity. Flowers, embryonic organs, germinating seeds, 

 buds, &c., appear to respire more vigorously than full-grown roots, stems, or 

 leaves, assuming of course that external conditions remain constant. It will 

 be advisable at this stage to illustrate these statements by a few tables. 



According to Aubert (1892, p. 375) the following plants absorb hourly the 

 following amounts of oxygen (in ccm.) per gram of fresh weight, at a tempera- 

 ture of 12° to 15° C. :— 



Cereus macrogonus 

 Opuntia cylindrica 

 Opuntia maxima 

 Pltyllocadus grandiflorus 

 Sedum alburn 

 Sedutn acre 



