THE ASSIMILATION OF CARBON BY AUTOTROPHIC PLANTS. II 125 



leaves at — 2-4°C. A similar result was obtained by Jumelle (1892) in the case 

 of other plants at quite low temperatures ( — 30° to — 40°C.). According to 

 EwART (1896), however, such low temperatures must speedily cause the chloro- 

 plasts to become inactive. [The dependence of the decomposition of carbon- 

 dioxide on temperature has been investigated very accurately by Matthaei 

 (1904). This author finds that the minimum lies near - 6°, the optimum about 

 37°, and the maximum about 43° ; the rise to the optimum is very rapid, and 

 the fall beyond that point still more sudden.] 



Although we have left the consideration of light to the last, it is not because 

 it is of little consequence ; on the contrary, it is of the utmost importance, 

 and may be best discussed after the other influential factors have been con- 

 sidered. The fact that assimilation is intimately dependent on the presence of 

 light may be proved with perfect ease, and branches of Elodea lend themselves 

 readily to such experimental treatment (p. no). [Very often the method is 

 employed of partially covering a foliage leaf with pieces of cardboard, when the 

 necessity for light in carbon-dioxide decomposition is shown remarkably clearly 

 by the non-formation of starch in the darkened regions. Haug (Bot. Gaz. 

 1903) has recently advanced some objections to this mode of experiment.] The 

 evolution of bubbles of gas which takes place with a certain rapidity when Elodea 

 is placed in a bright window decreases markedly when we withdraw the plants 

 to the back of the room, and ceases entirely before what we should term ' dark- 

 ness ' ensues. The facts already referred to have not, however, been strictly 

 speaking completely substantiated. In each green cell, as we have already seen, 

 carbon-dioxide is formed during respiration as well as decomposed during assimi- 

 lation. Respiration, however, may be said to be quite independent of light, 

 indeed it goes on with equal activity in feeble light as in direct sunlight. There 

 must, therefore, be a certain intensity of light under which as much carbon- 

 dioxide is decomposed in the process of assimilation as is formed in the process 

 of respiration, and then, of course, no air-bubbles at all will escape from Elodea, 

 and even the specially sensitive bacterium method fails to show the existence 

 of any assimilation. It is only by quantitative chemical methods that assimi- 

 lation can be proved to occur, provided the amount of respiration in the 

 parts concerned is known. When the intensity of light is still further reduced 

 assimilation manifests itself only through a diminution of the amount of respira- 

 tion, and comes to an end entirely when respiration ceases to make itself apparent. 



More exact estimates as to the minimum intensity of light which can still 

 bring about decomposition of carbon-dioxide are not forthcoming, and it is to 

 be expected that individual species will exhibit differences in this respect. It is 

 only too well known that most plants do not thrive in a room, and this is due 

 for the most part to the indifferent lighting of our houses. Since several plants, 

 such as Clivia and Aspidistra, do thrive in rooms, one might conclude from 

 that, that the minimum light intensity required is less in their case than in 

 that of others ; but it has been shown that in these, as in other shade-loving 

 plants, there is an entirely different reason for their power of endurance of weak 

 light, viz. their reduced respiratory activity, on account of which less organic 

 material is lost and there is less necessity for reconstruction. 



Further we are not accurately acquainted with the precise way in which 

 assimilation is dependent on the intensity of light. All researches agree on 

 one point, viz. that assimilation of carbon is approximately proportional in 

 amount to the intensity of light ; it is questionable, however, whether this 

 is the rule with higher intensities. Reinke (1883) found that the amount 

 of assimilation was the same whether the plant was illuminated by direct sun- 

 light or whether a lens with a magnifying power of 60 was used. But, as 

 Pfeffer has noted (Physiol, ist ed. I, 209) it may be easily imagined that 

 a further increase in assimilation, following on increase of light, is impossible 



