LIFE AND LIGHT 29 



sesses means of adapting itself to the wide variations occurring 

 diurnally and seasonally in these outer conditions. 



The natural amount of carbon dioxide present in atmospheric 

 air or water in equilibrium with atmospheric air appears to be the 

 best for the growth and development of plants, although it is not 

 by any means that at which for short periods the photo-synthesis 

 proceeds at the most rapid pace. Results of various observers 

 differ as to the correspondence between amount of carbon dioxide 

 in the air or water of the environment and the rate of photo- syn- 

 thesis; thus Kreusler 1 found that the rate was nearly doubled 

 with seven times as much carbon dioxide as in ordinary air, and 

 with 1 per cent. i.e., about thirty-five times as much as atmo- 

 spheric air 'there was only two and one-third times as active 

 photo- synthesis as in air, and this formed about a maximum. 

 Kreusler used continuous lighting with an electric arc light of 1,000 

 candle-power at a distance of 31-45 centimetres. 



Brown and Escombe, on the other hand, using sunlight in two 

 parallel experiments with sunflower leaves, exposed to like condition 

 of sunshine but with varying percentages of carbon dioxide, found 

 that up to seven or eight times the atmospheric amount of carbon 

 dioxide the activity rose in linear proportion, so that approxi- 

 mately seven or eight times as much carbon dioxide disappeared 

 when the partial pressure was correspondingly increased. This 

 condition of affairs obtained only for shorter intervals, for the 

 authors discovered that plants grown in an atmosphere richer in 

 carbon dioxide than normal, put on less dry weight than in a normal 

 atmosphere; the area of green leaf developed was smaller and the 

 colour of a deeper green. 



They repeated their work in a large-scale experiment conducted 

 in a greenhouse, which could be closed airtight and filled with air 

 containing known amounts of carbon dioxide in one half, the other 

 symmetrical half containing ordinary air. Under such conditions 

 they found that after some weeks' exposure the growth was affected 

 in an extraordinary manner by this plenitude of carbon dioxide. 

 The chief results were that in many cases the leaves dropped off 

 and were replaced by smaller ones of a deeper green ; the internodes 

 became much shorter and developed more rapidly, and there was a 

 profusion of axillary shoots, so that the plants from the artificial 

 atmosphere, which were of about equal height with the controls 



1 Landwirth, Jahrbiicher, 1885, Bd. xiv. ? p. 913. 



