28 LIFE AND LIGHT 



glycerides in the fatty depots of the connective tissue, or 

 elsewhere. 



If a protein molecule existing in colloidal complexity in the living 

 cell can take up molecules of hexose, and from energy obtained from 

 intercurrent reactions linked on at neighbouring positions of the 

 colloidal complex can fashion these into a portion of itself first, and 

 then turn them out into detached and growing granules of starch 

 or of fat, as we know experimentally it can, then it does not seem 

 improbable that the colloids of the chloroplastid can similarly 

 attach carbon dioxide and water molecules, and by means of the 

 light energy, causing molecular swingings or reinforcing them, in 

 neighbouring positions on the big colloidal molecule, can detach 

 oxygen and formaldehyde or similar groups. 



This view appears to the author more probable than the view 

 of free detached formaldehyde, for this if formed would immediately 

 unite to protein before it could be condensed to hexoses, and of all 

 aldehydes formaldehyde is most unlikely to be present, because 

 it would enter into fatally stable union with the protein instead of 

 retaining a labile easily altered form for further change under the 

 action of the light vibrations. 



Priestley, the discoverer of oxygen, first showed that green 

 plants exposed to light produced oxgyen, and the meaning of this 

 discovery was appreciated and further study devoted to the subject 

 by another British philosopher, Ingenhouze, 1 who as early as 1779 

 pointed out that the under surface of the leaf was most concerned 

 in the output of oxygen, that very young or very old leaves gave a 

 diminished yield, that all plants poisoned the air during the night, 

 and, if they were in shadow, during the day also, and that mosses 

 also, but not fungi, produced oxygen. At a later period in these 

 earlier days of the study of photo- synthesis much exact knowledge 

 was added by the labours of Senebier, Saussure, and Mohl, and by 

 Sachs, who first appreciated truly the function of the chloroplast. 



The rapidity with which photo-synthesis occurs depends upon 

 several factors such as intensity of illumination, concentration of 

 carbon dioxide in the atmosphere of air or water surrounding the 

 plant, and temperature of the surroundings. The plant also in the 

 orientation of its leaves, amount of opening of the stomata or minute 

 openings leading to its air vessels which form the respiratory system, 

 and degree of development of chlorophyll or other pigments, pos- 



1 " Experiments upon Vegetables." London, 1779. 



