94 THE PHYSIOLOGY OF STOMATA. 



be regarded as a compound grain, this condition being secondary, since the 

 whole is built up from several centers which appear distinct at other times. 

 The diameter of such a compound starch grain is nearly as great as that of 

 the nucleus and one-third to one-half the transverse measurement of the 

 guard-cell. Their size, together with their number, give the interior of the 

 cell a crowded appearance which often makes the observation of the nucleus 

 difficult. After sunrise, during a period of 4 to 7 hours, according to the 

 season, and, as I believe, according to the temperature, the amount of starch 

 is gradually reduced till a minimum is reached, which may mean its entire 

 absence, or a small quantity. In any event, the minimum amount is so 

 small that the plastids come to be comparatively insignificant in size, and 

 often lie in profile on the periphery of the cell, as seen from above (see plate 

 6, fig. 2, of stomata taken at 8 h 30 m a. m.). The difference in the amount of 

 starch is less marked when lower temperatures prevail. Indeed, I have found 

 it rather difficult, on cold days in early spring, to be assured that any differ- 

 ence at all was observable, but this would be expected under such conditions. 

 At this time the iodine test results in a brown stain of the plastid ; and when 

 applied with great care, in the event that starch is present, this will be 

 revealed as minute granules lying separately within the plastid. When the 

 material is boiled in the iodine solution the swelling of the starch granules 

 results in a lakey appearance of the plastid, best represented by a medium 

 shade of the "neutral tint" of water color of Windsor & Newton. 



The period of minimum starch content lasts, according to my notes made 

 in July, 1905, for 3 or 4 hours, this probably varying with the temperature. 

 At its close the granules increase in size until the maximum is again reached. 

 This occurs in a comparatively short time when once begun. As the middle 

 of the afternoon is approached, the amount of starch is obviously increased, 

 until at about 5 o'clock the maximum is nearly if not quite reached. 



During the night there seems to be no change, or if any occurs it is inappre- 

 ciable. This accords with the movements of the stomata during the same 

 time and, in fact, the correlation between the stomatal action and the varia- 

 tion in the amount of starch in the guard-cell is so constant and striking under 

 all circumstances that the former may well be now described, after which we 

 may return to consider the other cell contents. 



STOMATAL MOVEMENT. 



At sunrise the stomata are all nearly or quite closed (plate 7, fig. 1). 

 There is seldom to be found a complete uniformity in the size of aperture. 

 Measurements, as seen by the accompanying tables and notes, show that 

 there is a variation of size between o and 4 micra, transverse pore measure- 

 ment. Often the variation is much less, and cases have been observed in 

 which only, say, 5 per cent of the stomata show a sensible opening. The 



