1922-23.] BOTANICAL SOCIETY OF EDINBURGH 169 



" window " does have some effect on the total amount of light 

 reaching the deeper tissues. 



In this connection, the conclusions given below were based 

 on the method employed by Sachs. This experimenter 

 removed, in the morning, portions of leaves of equal area, 

 or actual halves, dried these and then weighed. The remaining 

 portions or halves were exposed all day and in the evening 

 similarly dried and weighed. Increase in weight indicated 

 gain due to photosynthesis. 



In the cylindrical leaves of S. gonocladus, surface areas 

 cannot readily be compared, but it was found that if leaves 

 of equal thickness are carefully selected, cylinders can be 

 punched out of them the weights of which are approximately 

 the same, the probable error of the mean weight of the samples 

 taken being less than 1 per cent. 



Plants of S. gonocladus were thus selected with leaves of 

 similar size. Some of the leaves were left in their normal 

 condition and in an equal number the " windows " wer6 

 blocked by painting them with Indian ink. The plants were 

 then placed in the dark until the leaves were found to be starch 

 free. Cylinders were then punched out of half the normal 

 leaves selected and from half of those with the " windows " 

 blocked. These were weighed, and dried at 100° C. until the 

 dry weight remained constant. The remaining leaves were 

 exposed to light for varying lengths of time, after which drying^ 

 and weighing was carried out as before. 



In all the experiments performed, whether sunlight or 

 artificial light was employed, it was found that the gain in dry 

 weight in the leaf with the " window " blocked was less than 

 the gain in the normal leaf. The experiments suggest, in 

 fact, that the translucent tissue in the leaf is used by the plant 

 as a means of illumination. 



In conclusion, I have to express my thanks to Professor 

 Wright Smith, Royal Botanic Garden, Edinburgh, for his 

 kindness in providing facilities for the experiments carried 

 out, and also to Dr. Graham and to Mr. L. B. Stewart for 

 drawing my attention to the peculiarity in the structure of 

 the leaf and for suggesting an experimental inquiry into its 

 significance. 



