Mason — Preliminary Notes on the Carbohydrates of the Musci. 27 



3. In P. commune and S. cyvibifolium the hexoses appear to be the 

 chief form in which the carbohydrates descend the stem. 



In reference to the remarks made above concerning the necessity of a 

 high concentration of hexoses in order that sucrose be synthesised under the 

 influence of invertase, it may be pointed out that though the experiments 

 quoted in this work exclude the possibility of a high concentration for the 

 whole leaf, yet they do not demonstrate the absence of a localized high con- 

 centration. This local concentration might exist in the chloroplasts, possibly 

 as a film in contact with the chlorophyll. That this is not so is shown by the 

 following considerations : — 



1. The chloroplast would require a selectively semi-permeable membrane 



to permit the sucrose to diffuse and yet retain the hexose ; 

 experiments on starch- formation from hexose solutions negative 

 this. 



2. In the chloroplast. or that region of it where the hexose concentration 



is supposed to be present, there would be approximately 1 per cent, 

 sucrose, and 99 per cent, hexose (the equilibrium percentages of 

 these sugars in the presence of invertase (Visser 12)); this would 

 involve (as analysis has shown) a diffusion of sucrose from the rest 

 of the leaf into the chloroplast ; since more than 1 per cent, of 

 sucrose is present throughout the leaf. 



3. In a lamella of P. commune composed of five tiers of cells, in order that 



sucrose may continue to diffuse away a high concentration is 

 rendered inevitable in the uppermost cells, but a high concentration 

 of sucrose in the cell prohibits the sugar formed in the chloroplast 

 from diffusing out into the vacuole and cytoplasm ; this banking 

 up of sucrose involves a higher concentration in the region of 

 hexose concentration than 1 per cent., and this would exclude 

 further synthesis under the influence of invertase ; but the 

 energy entering the chloroplast would ensure a continuation of 

 hexose-formation, and if this was unable to proceed to sucrose 

 and so be removed, a banking up of hexose would occur, and so 

 on through the different stages in the synthesis of the hexoses, 

 till at length a stage would be reached where the energy entering 

 the chloroplast would be unable to carry on the process. 



If we substitute a high concentration of sucrose for hexose in the 

 chloroplast, we escape all the above difficulties. It should be noted that 

 though invertase has been shown to be present in the leaf, yet it has not yet 

 been demonstrated that it is present in the plastid. 



