408 



Dr. H. T. Brown and Mr. F. Escombe. 



[Apr. 28 r 



the effect is probably indicated by the experiments on leaves described 

 in the early part of the paper, where it was shown that the amount 

 of photosynthesis in the leaf lamina is, within certain ill-defined limits y 

 a function of the partial pressure of the C0 2 in the surrounding air. 



In the first series of experiments in the greenhouse, where this partial 

 pressure was maintained at about three and a-half times the normal, 

 the plants for a certain limited period must have been manufacturing 

 carbohydrate material within their chloroplasts at least three and a-half 

 times faster than those in normal air, and, although this rate of photo- 

 synthesis would perhaps not be maintained for very long, yet there 

 would always be a general tendency for the carbohydrate supply in the 

 leaves to be kept up to a higher point than in the controls grown in 

 ordinary air, a fact which was shown by the leaves of set B always 

 being gorged with starch. 



Since it is quite certain that this increased photosynthesis does not 

 to any material extent contribute to the increase of dry weight of the 

 plants, we can only conclude that the transformation, translocation,, 

 and general metabolism of the leaf-reserves under these conditions- 

 cannot keep pace with the increased tendency to produce an extra 

 amount of plastic material from the atmosphere. Moreover, it is clear 

 that the whole mechanism of the plant on which normal nutrition 

 depends has its parts so completely and accurately correlated that any 

 slight increase in the composition of the surrounding air which favours 

 increased photosynthesis destroys the adjustment of the various parts- 

 and results in a more or less abnormal development of the plant. That 

 any such disturbance of the economy of the plant should profoundly 

 modify the reproductive functions, might perhaps have been expected. 



It is somewhat remarkable to find that all the species of flowering- 

 plants, without exception, which have been the subject of experiment, 

 appear to be accurately " tuned " to an atmospheric environment of 

 3 parts of C0 2 per 10,000, and that the response which they make to 

 slight increases in this amount, are in a direction altogether unfavour- 

 able to their growth and reproduction. It is not too much to say that 

 a comparatively sudden increase of carbon dioxide in the air to an 

 extent of but two or three times the present amount, would result in 

 the speedy destruction of nearly all our flowering plants. 



To a certain extent we may regard the facts recorded in this paper 

 as indicating that the composition of our atmosphere as regards its 

 carbon dioxide, has remained constant, or practically constant, for a 

 long period of time, but they leave altogether untouched the question 

 of any variations of a secular kind. All we are justified in concluding 

 is that if such atmospheric variations have occurred since the advent 

 of flowering plants, they must have taken place so slowly as never to 

 outrun the possible adaptation of the plants to their changing condi- 

 tions. 



