RESPIRATION 195 



be inhibited more readily than the respiratory function by the employment of such 

 poisons ; for Ewart (1896) was able to inhibit assimilation for a certain time 

 by the use of ether vapour, although respiration continued and the organism 

 remained alive. Such experiments would, however, have increased value if it 

 were possible by etherization to inhibit assimilation completely whilst leaving 

 respiration entirely unaffected. Bonnier and Mangin (1886) have attempted 

 to do this, but their results, taken in relation with those of other observers, are 

 open to criticism, and may indeed have been obtained rather by good fortune than 

 otherwise. It has often been observed that weak etherization accelerates 

 respiration (Elfving, 1886; Johannsen, 1896; Morkowin, 1899), whilst strong 

 etherization, by killing the cells, inhibits it. That respiration may continue 

 constant in leaves placed in a narcotic condition is possible, but, as we have said, 

 it is only by chance that such a result is obtained, and experiments carried out 

 with leaves of the same kind do not confirm Bonnier and Mangin's results. 



Another method of proving directly that respiration takes place in green 

 cells exposed to light, which was for long believed to be effective, was that 

 employed by Garreau (185 1). Garreau showed (by means of baryta water) that 

 demonstrable traces of carbon-dioxide always escaped from illuminated branches, 

 and he believed he was dealing in that case with carbon-dioxide formed in the 

 process of respiration and escaping from the chloroplasts. Blackman (1895) 

 showed, on the other hand, that this excretion of carbon-dioxide was very 

 improbable, and that it was incapable of demonstration if one experimented 

 exclusively with chlorophyll-containing cells and rejected all peduncles and stems 

 not possessing green colour, a precaution which Garreau failed to take. 



Although direct proof of continuous respiration in illuminated green 

 cells cannot be obtained, indirect evidence is available. We may observe 

 not infrequently in green cells during active assimilation a continuance of proto- 

 plasmic movement and growth, two phenomena which are impossible in the 

 absence of respiration. In general, it is correct to assume that respiration takes 

 place in light to as great an extent as in darkness. The assimilatory activity 

 of a foliage leaf must be estimated not only by direct measurement of the 

 amount of carbon-dioxide taken up from without but also by estimating the 

 amount of carbon-dioxide which is produced during the same time by respira- 

 tion, but which does not escape from the plant because it is at once employed 

 in assimilation (compare Lecture X, p. 124). 



Since, also, the respiratory gaseous exchange is, under normal conditions, far 

 less intense than the assimilatory gaseous exchange, it follows that in the long run 

 a plant organmaysufferfromscarcity of oxygen or from the presence of injurious 

 quantities of carbon-dioxide. Carbon-dioxide, when in sufficient accumulation, 

 undoubtedly interferes with the essential functions of the plant, so that removal 

 of the gases arising in the course of respiration may be considered as absolutely 

 necessary. The removal of these is a simple matter in the assimilating leaf. If 

 an accumulation of carbon-dioxide has taken place in it during the night, in the 

 morning it is at once removed on the commencement of assimilation. The 

 abundant intercellular spaces with their openings, the stomata, are well adapted 

 for the promotion of thorough aeration. The gaseous exchange is conducted with 

 greater difficulty in colourless subterranean organs ; but here also the individual 

 cells, by means of intercellular spaces, are, generally speaking, favourably 

 situated for giving off and taking up gases. Since these organs, however, have 

 no direct exit passages the gases must either travel long distances to reach the 

 aerial stomata or must escape by diffusion through the cuticle. Without doubt 

 the cuticle of subterranean organs offers much less opposition to such a diffusion 

 than does that of the foliage leaf ; we have already seen how permeable it is 

 to water, and it would appear to be equally permeable to carbon-dioxide. So 

 far also as the oxygen is concerned, that gas, owing to vigorous partial pressure, 



o 2 



