INFLUENCE OF EXTERNAL CONDITIONS ON PHOTOSYNTHESIS 337 



Owing to reactions of this character, as well as to the more or less 

 marked powers of accommodation which all plants possess, it is often difficult 

 or impossible to establish any accurate numerical relationship between the 

 external conditions and the changeable vital activities of the plant, for at 

 a minimal or a maximal temperature the assimilation of carbon dioxide, the 

 processes of growth, &c., continue feebly for a time, but ultimately cease. 

 The following data are all estimations made under the immediate action 

 of the changed conditions, and hence are only approximately accurate. 

 Within certain favourable limits, however, the chloroplastid rapidly ac- 

 commodates itself to altered conditions. 



Influence of the external conditions Temperature. The curve of 

 assimilation rises as the temperature increases, and remains fairly constant 

 at an optimum ap- 

 proximating to that 

 for growth, as is shown 

 in a special case in 

 Fig. 50. Above this 

 optimum the assimi- 

 latory curve falls again, 

 whereas the respira- 

 tory curve continually 

 rises (Fig. 50 and Sect. 

 114). The difference 

 between the gain of 

 organic substance by 

 photosynthesis and its 

 loss by respiration 

 hence rapidly dimi- 

 nishes as the temperature rises beyond the optimum, and although in 

 the case given in Fig. 50 assimilation surpasses respiration even to the 

 last, it is often the case that leaves with abundant chlorophyll under brilliant 

 illumination may produce more carbon dioxide at or above 40 C. than 

 they can decompose (Kreusler, I.e., 1890, p. 663). If the plants are exposed 

 for some time, the same result may be produced at an even lower temperature. 

 Thus Ewart found under such circumstances that at 37 C. to 38 C. he 

 could not detect by the bacterium method any evolution of oxygen from the 

 chlorophyllous cells and tissues of Aspidium, Mimosa, Chara, Elodea, Sela- 

 ginella, Oxalis, Cystopteris, Metzgeria, Orthotrichum, Parmelia, Dicranum, 

 Bryum, &c., even under optimal illumination. It is very probable that 

 prolonged exposure to a high temperature not only exercises a depressing 

 effect upon the assimilatory powers of the chloroplastids, but also acts as 

 an accelerating stimulus and produces an abnormal increase in the respiratory 

 .activity (Sect. 104). ' The true curve of assimilation can only be obtained by 



1,6" tt,3 



M6" 25 23,3*33 37,3" tt.l* 



FIG. 50. Assimilation and respiration in a leafy branch of Rubus fruti- 

 cosus at different temperatures (after Kreusler). When assimilating, the 

 branch is surrounded by air containing 0-3 per cent. CO 2 , and is illuminated 

 by electric light ( = bright diffuse daylight). By adding the height of the 

 respiratory curve to that of the assimilatory one the actual amount of CO 2 

 assimilated is obtained. 



- C0 2 assimilated I per hour per sq. metre of leaf-surface. 



...... CO 2 produced by respiration! ^ 



