RESPIRATION 201 



compounds were indeed the final products of respiratory metabolism, we might 

 say that the green plant differs essentially from the higher animal in being able 

 to make use both of its nitrogenous and non-nitrogenous metabolic products 

 as nutritive materials once more. Since animals do not possess this power, 

 they may in that respect be contrasted with plants. 



Respiration, like all other functions of the organism, depends on the ex- 

 ternal factors of which we have already had occasion to speak. Light, according 

 to KoLKWiTZ (1899), produces a feeble rise in respiration — at least in Fungi — 

 although it is impossible to say whether its action is purely chemical (by decom- 

 position of certain organic acids) or whether its influence is more far reaching and 

 affects the protoplasm. Since in other cases, however, a reduction in the respira- 

 tion has been observed when the plant is illuminated, the question cannot be 

 considered as in any way settled. More recently, Maximo w (1902) has only 

 partially confirmed the results obtained by Kolkwitz. One thing at least is 

 certain, that light has no essential influence on respiration ; heat, on the other 

 hand, is of fundamental importance. Although the curves expressive of the 

 dependence of the majority of physiological processes on temperature resemble 

 very closely that of assimilation (p. 124), exhibiting between the minimum and 

 maximum a pronounced optimum, this last datum is not quite determined for 

 respiration. Pfeffer holds the view that increase in the intensity of 

 respiration is concomitant with increase of temperature until the latter begins to 

 influence injuriously all the vital processes. The plant must be permanently 

 injured by temperatures which bring about a diminution of respiration. If, 

 however, we accept the results arrived at by Ziegenbein (1893) we must 

 acknowledge the probable existence of an optimum temperature for respiration. 

 Ziegenbein finds that the intensity of respiration (measured by the amount 

 of carbon-dioxide excreted (in mg.) per'cent. of fresh weight) depends on the 

 temperature in the manner illustrated by the following table : — 



Although one is compelled to hold that all temperatures above 45° are cer- 

 tainly injurious, and that the observed reduction in respiration at 50°, &c., 

 must be regarded as the result of the plant's pathological condition owing to the 

 excessive temperature, it is difficult to believe that a temperature of 40°, 

 which causes a diminution of respiratory activity in the last two objects experi- 

 mented on, operates in this way. Ziegenbein, at least, fails to offer exact proof 

 that this was not the case. Should an optimum temperature be established for res- 

 piration later on, it must lie in each case extremely near the maximum. Further 

 evidence as to the optimum temperature is given by Kunstmann (1895) for 

 Fungi, and Stoklasa (1903) for beetroot. The position of the minimum has 

 been often worked out and it has been found that it lies considerably below 

 0° C, in the case of lichens, for example, about -10° C. (Jumelle, 1892). The 

 increase in respiration near the maximum temperature is often very marked ; 

 it reaches, for example, according to Clausen's experiments (1890) on germin- 

 ating wheat, to as much as eleven times, and in the case of lupins to sixteen 

 times what it is at 0° C. The fact that respiration is accompanied by a 'pro- 

 duction of heat need only be casually referred to here, since that phenomenon 

 will require closer consideration in another place (Lecture XXXI). 



In regard to the influence of materials on respiration we may consider 

 first of all water, which has no specific influence on respiration, but is important 

 only in so far as it is one of the general vital conditions. Respiration 

 ceases entirely in seeds and perfectly dry parts of plants, in mosses, lichens, &c., 

 which can tolerate complete desiccation, continuing alive during drought without 



