Considered in Relation to its Function. 
409 
in a very thin layer. Then the thinner the layer in which a definite amount 
of chlorophyll is disposed, the greater the amount of energy set free at any 
part of that layer. Consequently the efficiency of the chloroplast in 
bringing about the photolytic decomposition of carbon dioxide must depend 
on the thickness of the layer. 
As some recent work carried out by one of us, in conjunction with 
Mr. Usher , 1 had confirmed the impression that the thickness of the layer 
was of importance, it seemed desirable to repeat some of these previous 
observations, and to see if it were possible to come to an independent 
conclusion. 
The Microscopic Structure of the Chloroplast. 
Plants containing relatively large chloroplasts, generally either Seta - 
ginella Martensii, S. Kraussiana, or Chlorophytum elatum y were taken, and 
various attempts were made to elucidate their structure. 
In the case of the Selaginella y it was difficult even when using Zeiss 
apochromatic immersion objectives (J and ^ ) and an achromatic con- 
denser, to discover any signs of heterogeneity of structure ; but with 
Chlorophytum such indications were very marked. The chloroplast of the 
latter plant clearly seemed to consist of a network with chlorophyll in the 
meshes. Furthermore, in many cases it was possible to obtain optical 
1 Usher and Priestley, Proceedings of the Royal Society B, vol. lxxviii, 1906. 
