OUTLINES OF THE PHYSIOLOGY OF NUTRITION 223 



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Generally speaking, they open in light and close in 

 darkness, i.e. they open when the conditions are 

 favourable for assimilation. Thus transpiration and 

 assimilation go on together, an arrangement which is 

 evidently highly expedient, for the current set up by 

 transpiration brings to the leaves the water and 

 mineral matter without which assimilation cannot 

 take place. 



A few words must be said as to the mechanism of 

 the stomata, though the subject is too difficult to be 

 discussed fully here. As 

 a general rule, the 

 stomata open when the 

 guard-cells are turgid, 

 and close when they are 

 flaccid. This can be 

 proved simply enough 

 in the case of large 

 stomata, such as we 

 find in many Monocoty- 

 ledons. A piece of 

 epidermis is removed 

 from a living leaf and 

 mounted under the 

 microscope, first of all 

 in distilled water. The guard-cells curve more and 

 more, and the pore between them opens to its full 

 extent (see Fig. 95, B)\ next we replace the water 

 by a solution of salt a strength of 2 per cent, 

 is sufficient. The curvature of the guard-cells now 

 diminishes, and as they straighten themselves the 



FIG. 95. Stoma of Amaryllis, p, 

 pore. A, In 2 per cent, salt solu- 

 tion ; the guard -cells are flaccid, 

 and the pore closed. B, In distilled 

 water ; the guard- cells are turgid, 

 and the pore wide open. Magnified 

 about 150 diameters. (E. S.) 



