492 SECTIONAL TRANSACTIONS— K. 



SECTION K.— BOTANY. 



Thursday, August 18. 



Presidential Address by Prof. W. Stiles, F.R.S., on The general physio- 

 logy of the plant cell and its importance for pure and applied botany 

 (lo.o). (See p. 213.) 



Joint Discussion with Section C (Geology) on The postglacial history of 

 Fenland (11. 15). See under Section C. 



Afternoon. 

 Excursion to Roman Road and Gog Magog Hills. 



Friday, August 19. 



Joint Discussion with Section D (Zoology) on The mechanism of evolu- 

 tion (lo.o). See under Section D. 



(concurrently with above session.) 



Prof. F. J. Lewis. — The physical nature of the outer surf ace of the cell walls 

 of the mesophyll of the leaf (lo.o). 



The outer surface of the mesophyll cell walls is unwettable by water 

 but wettable by hydrocarbons. Infiltration of the leaf with dye solutions 

 shows that dyes with an acid chromophore pass in with the water and fill 

 the interspaces. Infiltration with dyes having a basic chromophore results 

 in the adsorption of the dye on the surface of the walls at the point of entry 

 while the water alone passes on and fills the interspaces. On exposure of 

 the mesophyll the dye adsorbed on the wall is found to be insoluble in water. 

 The eflfect of a fatty acid such as sodium taurocholate is to render the 

 surface of the walls wettable, but this action only takes place within certain 

 /)H limits. 



Dr. W. R. G. Atkins. — The measurement of light in relation to plant 

 growth and distribution (10.30). 



Attention is drawn to the necessity of a thorough study of the instruments 

 used in measuring light, so that the relation between current and intensity 

 is known. The optical properties of diffusing surfaces and colour filters 

 must also be considered, as well as the angular distribution of the light. 

 The daily and seasonal changes in vertical illumination have been measured, 

 and the variations from year to year noted for eight years. Shaded habitats 

 are best rated by the ' daylight factor,' though the ' colour factor ' also 

 deserves consideration. Penetration of light into the sea results in an 

 alteration both in intensity and colour. The effect on diatom cultures has 

 been studied and reveals an optimum depth for photosynthesis. The. 

 explanation of the disappearance of Zostera marina which attributes this to 

 lack of sunshine resulting in attack by parasites is disproved by an adequate 

 study of the meteorological records. 



