OUTLINES OF THE PHYSIOLOGY OF NUTRITION 225 



selves. Now, when exposed to light, the chlorophyll- 

 corpuscles of the guard-cells become active ; they form 

 new organic substances, which pass into solution in 

 the cell-sap. Hence the concentration of the cell-sap 

 is increased, water is absorbed by osmosis from the 

 neighbouring cells, and the turgidity of the guard- 

 cells, which is necessary to effect their opening, is thus 

 attained. 



But that this is not the whole explanation is shown 

 by the fact that stomata sometimes open in light, even 

 in the absence of carbon dioxide. 



The aqueous vapour transpired by the stomata 

 comes immediately from the intercellular spaces, into 

 which it has been given out by the mesophyll-cells, 

 especially those of the spongy parenchyma. As 

 transpiration goes on, the cells of the mesophyll con- 

 stantly tend to become poorer in water, and they 

 make good their loss by taking up fresh supplies from 

 the water-conducting tracheides of the vascular bundles, 

 by which every part of the leaf is traversed. We see 

 now how appropriate is the comparison which has been 

 drawn between the network of bundles in the leaf and 

 the system of irrigation channels in a water meadow. 



When transpiration is active, most of the tracheae 

 become comparatively empty, the water forming a series 

 of short columns, the spaces between which only con- 

 tain watery vapour and an extremely small quantity of 

 air, so that the pressure is very much below that of 

 the atmosphere, and often becomes so reduced that we 

 may speak of it as an approximate vacuum. From this 

 fact alone it is quite evident that the pressure from 

 the root does not keep pace with the transpiration 

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