70 EFFECT OF PHYSICAL FACTORS AND PLANT CONDITIONS. 



that the plot of alfalfa plants, for instance, having the highest 

 average of water in their leaves during each 24-hour period would 

 exhibit the greatest growth and would produce the most dry matter. 

 This again has not been found to be the case. The plants of plot 



2 used in series 32, which was discussed in connection with water- 

 content, showed a higher average per cent of leaf -water than did 

 the plants of plot 3, but the plants of the latter showed the greatest 

 increase of dry matter during the two weeks' observation. The 

 greater decrease of leaf-water during the day of the plants in plot 



3 can not be assumed to have interfered in any manner with their 

 functioning. Hence, it may safely be said that fluctuations of 

 leaf-water are normal and universal. When not so great as to 

 induce closure of the stomata, they do not inhibit or in any other 

 manner affect photosynthesis, translocation, or other functions of 

 the leaf. Such fluctuation may be said to occur only in the working 

 margin of the leaf-water. 



Changes of leaf turgor offer the best explanation regarding the 

 mechanism of mid-day closure and night opening. It has always 

 been found that mid-day closure occurs when the leaf-water has 

 been reduced to a point which is the safe minimum for a given water- 

 content. The stomata do not reopen until the per cent of water 

 rises once more above this point and the leaf again has a margin 

 with which to safely operate. It is not known definitely whether 

 this is due to the effect of loss of turgor causing reconversion of 

 starch and thus producing closure, or whether evaporation concen- 

 trates the sap in the adjacent cells to a sufficient degree to cause 

 exosmosis from the guard-cells. Inspection of the relation between 

 the starch index curve and stomatal curve for series 10 (fig. 29) 

 would indicate that both are possibly involved. Night opening 

 occurs only in those leaves in which turgor is recovered faster than 

 starch is stored in the guard-cells. It therefore seems plausible 

 that the sap of the parenchymatous and adjacent epidermal cells is 

 diluted more rapidly by this increase of turgor than that of the 

 guard-cells is by the removal of sugar. This would then produce 

 a relatively higher concentration of sap in the guard-cells and con- 

 sequent opening of stomata, in spite of the fact that such concen- 

 tration was decreasing. The great difference found by Iljin (1914) 

 between the concentration of sap in the guard-cells and the sur- 

 rounding epidermal cells would make it impossible for night opening 

 to occur in this manner, but his results are undoubtedly too high. 

 Wiggins (1921), repeating this work with apparently much greater 

 care, found considerably less difference. Because he failed to disrupt 

 the neighboring epidermal cells, however, the solutions producing plas- 

 molysis of the guard-cells were probably diluted by passing through 

 these adjacent cells, and hence his findings are also too high. It seems 

 hardly possible, for example, that the guard-cells can lose their turgor, 



