120 THE PHYSIOLOGY OF STOMATA. 



ADDITIONAL EXPERIMENTS. 



Experiment 151, Fouquicria splcndens. — -After 20 hours in total darkness, 

 the stomata were open about 3 micra, some as wide as 9 micra. 



On a shoot kept in constant darkness for 3 days young etiolated leaves 

 formed. The stomata of these leaves were open, though not widely, since 

 they were too young. 



After 5 days the mature leaves of the same shoot continued to show plenty 

 of starch in the guard-cells till the leaves commenced to drop off. Even 

 during abscission the starch is not removed from the stomata. 



Experiment 164. — After constant darkness for 15 hours, the stomata 

 measure, at 1 i h 2o m a. m., 2 to 6 micra. 



Experiments 163 and 165. — After 15 hours' darkness, the stomata are 2 to 

 4 micra. Scarcely any wholly closed. 



Experiment, Iris sp. — -A plant was etiolated for 30 days (June 12 to July 12, 

 1906) and at the end of this period both young and mature stomata were 

 found abundantly supplied with starch. No change in the form of the stoma 

 could be observed, either in this or some other experiments made with the 

 same species. 



The constant occurrence of a considerable amount of starch in the stomata 

 after so long an exposure to darkness, as here observed, and as noted also by 

 others elsewhere mentioned, appears to be contingent, in part at least, upon 

 the possession by the plant of a store of food material. 



Analogous is the occurrence of starch in the guard-cells on the underground 

 parts of the leaves of Brodhea, which are found abundantly supplied with 

 starch, and the stomata open (January 5, 1907). 



CONCLUSION. 



The conclusion may be drawn that the absence of light does not result 

 in the loss of starch by the guard-cells as by the chlorenchyma, nor in the 

 inhibition of its accumulation, if the necessary materials are available in the 

 leaf-tissues, or elsewhere. The possession of chlorophyll by the stomata of 

 many plants under complete or much prolonged etiolation (Sachs, MacDougal) 

 as in the Iris sp. studied, may be explained by the availability of starch in 

 storage organs, which are a source of energy for plants deprived of light. Thus 

 seedlings grown in feeble light or deprived of carbon dioxid have been shown 

 to transfer their starch from one point to another (Bohm, 1874; Schimper, 

 1880). 



The opening of stomata is not entirely inhibited when the usual stimulus 

 is removed and darkness is more or less prolonged, though the cause for such 

 movement is obscure, as was observed by Leitgeb and Francis Darwin. 

 Darwin criticized Leitgeb 's explanation that the opening is due to the moist 

 atmosphere, and justly. The same thing occurs at low humidities. It is, 

 however, not obvious how Darwin's own explanation, that the guard-cells are 



