ABSORPTION AND MOVEMENT OF WATER 149 



within a few seconds from blue to pink, the color remains 

 unchanged for an hour or more (with Tradescantia zebrina, 

 for four hours) in the paper in contact with the closed 

 stomata. In spite of the very minute size of the opening, 

 even at its fullest extent, the great number of stomata give 

 us some idea of the effectiveness of these gates at the en- 

 trances of the intercellular passages. Pfeffer,* quoting 

 figures as to the size and number of stomata, says the 

 effective area of the opening seldom reaches .0046 sq. mm., 

 but that the number of such openings is from 100 to 300 

 per sq. mm. Estimating 100 to the sq. mm. at the size 

 .0046 sq. mm., we see that nearly one-half the surface can 

 be opened. Assuming that the stomata are on one side of 

 the leaf only, and that on that side they have the propor- 

 tions just given, we see that roughly one-quarter of the 

 leaf-surface can be the free path for the exchange of gases 

 and vapors. 



The disparity between absorption and evaporation which 

 for physical reasons forces the stomata to close, is often 

 supplemented by the irritability of the protoplasm of the 

 guard-cells. Evaporation insufficient to produce any dimin- 

 ution in turgor visible as wilting may still be enough to 

 irritate the g^ard-cells into reducing their turgor by physi- 

 cal or chemical changes accomplished in the cells by the 

 living protoplasm. Thus various influences which cannot 

 bear directly upon turgor, but which can irritate the proto- 

 plasm, bring about the closing and opening of the stomata. 

 The statements of older authors regarding these influences 

 have been lately tested by Stahl,t and still more recently by 

 Francis Darwin,! from whose papers the following conclu- 

 sions are abstracted. The stomata of most plants are widest 

 open in bright light, less widely open or completely closed 

 in darkness. This one would in general expect, for photo- 

 synthesis in chlorophyll-containing cells ( and the guard-cells 

 contain chlorophyll) is most rapid in bright light. There 



* Pfeffer, W. Pflanzenphyeiologie, I., p. 177. Eng. transl., I., p. 195. 

 Compare also Brown (Fixation of Carbon, Nature, 1899), and pp. 45, 50 

 of this book. 



f 7. c. J 1. c. 



