GENERAL SUMMARY. 1 39 



PHYSIOLOGY OF THE GUARD-CELLS. 



There is in Verbena ciliata a fluctuation of the amount of starch within the 

 plastids of the guard-cells, accompanied by a complementary fluctuation of 

 the oil content. The origin and the conditions contributory to the formation 

 of the oil content have, not been determined, but such evidence as has been 

 afforded indicates that this substance is of secondary importance. The starch 

 content is at its minimum during the later hours of the morning, being reached 

 earlier in the warm weather. During the earlier hours of the morning this 

 material is dissolved and disappears from the plastids of the guard-cells, 

 beginning at sunrise. The increase to the maximum occupies the afternoon 

 and probably the earlier part of the night, the maximum amount being observ- 

 able in the hours of the night. The movements and periods of stasis of the 

 stomata are closely correlated with this appearance and disappearance of 

 starch, and this appears to be the case also in the ocotillo (Fouquieria splen- 

 dens) both in regard to the fluctuation of the starch content and the stomatal 

 movements. This behavior, and the extremely small amount of chlorophyll 

 present in the guard-cell plastids, together with the accumulation and remark- 

 able persistence of starch in the guard-cell during darkness, strongly suggest 

 the hypothesis that the physiological activities within the guard-cell are 

 quantitatively different from those of the ordinary chlorenchyma cell, a view 

 at variance with the conclusions generally held, but which are not accepted 

 by all students of the problem without objection. 



In detail, it would appear that the plastids of the stoma, which present 

 certain morphological differences needing further investigation, normally 

 accumulate starch under conditions during the continuance of which the 

 chlorenchyma plastids do not normally do so, namely, when exposed to the 

 blue end of the spectrum, to darkness, and to air devoid of carbon dioxid. 

 Beginning with an initial condition of the entire absence of starch, this sub- 

 stance has been observed to be formed, and to materially increase in quantity, 

 when photosynthesis was impossible. This phenomenon is dependent upon 

 the presence of available carbohydrates within the chlorenchyma, since it 

 is possible to prolong the conditions of experimentation sufficiently to induce 

 starvation of the stomata and a consequent reduced amplitude of move- 

 ment. In this case the stomata most favorably placed are last to lose their 

 starch content, and when again placed in the light reform their starch more 

 slowly than the chlorenchyma. It follows that the guard-cell plastid not 

 only is able to accumulate starch in the dark and without a supply of carbon 

 dioxid (as are also the chlorenchyma plastids under special conditions) , but 

 normally does so by drawing upon the general supply of the leaf. The func- 

 tion of the chlorophyll in the plastids within the stomata of many plants is 

 probably complementary to the secretion, otherwise than by photosynthesis, 



