202 METABOLISM 



exhibiting any obvious metabolism ; minute quantities of water enable the for- 

 mation of carbon-dioxide to commence at once (KoLKWixz, 1901). The substances 

 which are used in respiratory metabolism are naturally of greater significance, 

 and if they be present in insufficient quantity respiration ceases, as, for example, 

 when plants are kept for a long time in the dark. In Fungi, such as Aspergillus 

 (KosiNSKi, 1901), which accumulates no reserves, the withdrawal of the nutritive 

 solution makes itself evident at once in a reduction of respiration ; so long, 

 however, as the organism remains alive respiration does not cease entirely, 

 and if nutrients are supplied to the plant after cessation of the period of starva- 

 tion respiration at once increases. Under normal nutritive conditions, however, 

 the amount of respiration is by no means proportional to the quantity of 

 respiratory materials present, and this fact is of the utmost importance in 

 a theoretical consideration of respiration. 



Generally speaking, an increase in respiration may be noted if the plant 

 be subjected to injurious influences. Thus, for example, small doses of 

 certain poisons act in this way, and perhaps we may correlate this result with 

 the increased growth which results from the addition of such stimulants (p. 88). 

 Similar results are produced by anaesthetics and antipyretics (jACOBi, 1899), as 

 has been already noted (p. 195). Carbon-dioxide acts in a like manner if it accu- 

 mulates in excessive amounts, and the same result is produced as an after-effect 

 of high temperatures, high atmospheric pressure, wounds, &c. (RICHARDS, 1896). 



In conclusion we may note the effect of oxygen, the gas which is most con- 

 cerned in respiration. It is worthy of note that respiration is independent 

 within wide limits of the percentage of oxygen present in the air ; the partial 

 pressure of oxygen may be reduced or increased considerably in comparison with 

 the normal without at once influencing respiration . Hence the presence or absence 

 of indifferent gases, such as nitrogen, appears to be without significance, respiration 

 takes place in pure oxygen just as in ordinary air, in which the oxygen is reduced 

 to one-fifth of its volume; in both cases, the partial pressure of oxygen is, however, 

 the same (i. e. one atm.) Only after the pressure of pure oxygen is raised to 2-5 

 atmospheres, does there ensue a marked increase in respiration followed soon 

 after by a marked diminution until death takes place ( JOHANNSEN, 1885). The 

 fact that death always takes place under higher oxygen pressures is not due to 

 increase of respiration, since far greater increase can be reached by other means, 

 e. g. higher temperatures, without any evil effect following. Why a fatal effect 

 should result from an increased access of oxygen we do not know ; only this much is 

 known that the different types of plant-life behave very differently in this relation, 

 since all transitions occur between such plants as we have as yet alone studied and 

 organisms which become injured by pressures of oxygen far below that present 

 in ordinary air (compare Lecture XVII). 



Respiration, as already stated, is at first not affected by a re- 

 duction of oxygen pressure, and STICH (1896) showed that not until the 

 proportion of oxygen in the air was as low as 2 per cent., or even less, was there 

 any diminution in the amount of carbon-dioxide given off. Experiments on this 

 question are not at all easy to carry out because, as has long been known, carbon- 

 dioxide continues to be given off for a considerable time after the complete 

 exclusion of oxygen. In some plants (Vicia faba, Ricinus, &c.) this excretion 

 is not less intense than when oxygen is supplied ; in the majority of cases how- 

 ever it reaches only one-third to two-thirds of this value, and varies in the individual 

 plant, according to its developmental condition. The carbon-dioxide produced 

 in this oxygenless respiration arises from the same materials as are consumed in 

 ordinary respiration ; still it cannot arise from simple combustion but from 

 the splitting of organic bodies, resulting in the formation of both reduced and 

 completely oxidized bodies. Oxygen atoms wander within the molecules of 

 the respiratory materials in so-called intra-molecular respiration. When glucose, 



