10 THE PHYSIOLOGY OF STOMATA. 



extent of opening to the same scale. Roughly speaking, the transverse form 

 may be described as elliptical, but this is departed from in the acute apices 

 and flattened sides, the departure being greatest in regard to the apices in 

 the smaller openings and in the sides in the larger. 



The plastids of the guard-cells are numerous and very faintly green. The 

 size of the plastid varies with the amount of contained starch, and this doubtless 

 modifies the optical effect of the chlorophyll upon the eye. Even when the 

 amount of starch is low the amount of green color is very small, so that the 

 plastids appear almost colorless. The contrast between the chlorenchyma 

 plastids and those of the guard-cells is in this regard very striking, as also is 

 the difference in the origin and development of the starch granules. In the 

 chlorenchyma plastids the starch appears as small lenticular bodies, two to 

 three or four in number, but never large enough to modify the form of the 



plastids in which they occur. In the 

 guard-cell, however, the starch granules 

 are spherical, except for the contigu- 

 ous flattened facets caused by mutual 

 contact. They are, moreover, so large 

 that the plastids, when fully loaded 

 with starch, appear as a compound 

 starch grain, such as occurs in certain 

 cereals, while the matricial substance 

 can scarcely be detected. 



The application of Molisch's (1896) 

 potash test shows a pronounced reac- 

 tion, but while the plastids of the 

 chlorenchyma retain their individu- 



FiG.3.-Transverse sections of stomata of Fou^uie- alit those of the guard^Hg do not. 

 rta splendens. Two stomata, one closed and * 



the other open, drawn to the same scale. The This may be due to the Swelling of the 

 black square represents 40 micra on each side. gtarch &nd consequent disorganization 



of the plastids, which, being crowded together in this condition, would pro- 

 duce a general coloring effect rather than that of distinct bodies similarly 

 colored. 



The Molisch test may, however, be misleading, though this must be said 

 with a full appreciation of the care with which he has studied the reaction. 

 Thus, I have succeeded in getting a curious reaction in certain leaf -cells in 

 which no trace of chlorophyll was optically present, while in similar cells in 

 another region of the same leaf no reaction followed. For example, the guard- 

 cells of a species of Linum gave a reaction on one side of the leaf, while this 

 did not occur on the other. Epidermal cells also give an appearance of a 

 yellow-green solution, when there are no plastids observably green. 



