II. THE EFFECT OF PHYSICAL FACTORS AND PLANT 

 CONDITIONS UPON STOMATAL MOVEMENT. 



The effect of stimuli upon stomata has received by far the great- 

 est attention from investigators in this field. The problem is so 

 vast that no attempt was made to cover it in detail, but certain facts 

 have been brought out in the course of the experiments which aid 

 in explaining the behavior of stomata. Hence, the discussion is 

 confined to the previous series described in their relation to physical 

 factors, and to other series and experiments designed to afford an 

 explanation of some of the phenomena observed. It is in this con- 

 nection that greenhouse experiments are particularly valuable in 

 supplementing field observations. 



Some factors, such as light, have a direct effect upon stomatal 

 movement, others, like relative humidity, have an indirect effect by 

 acting upon the leaf as a whole, or like wind by increasing the effect 

 of some other factor. The rate of change in a factor may have an 

 effect when rapid, and little or none at all when slow. Minor changes 

 of a factor have little or no effect, except at the critical point. Thus, 

 changes in light intensity have no observable direct effect if they occur 

 above 50 per cent of maximum for the region, but produce correspond- 

 ing changes in stomatal movement in many plants growing where 

 the light is less than 10 per cent. There is always a lag between the 

 impact of a factor and the resultant effect upon the stomata. This 

 lag is of variable length and depends largely upon temperature and 

 degree of impact, but often upon other conditions as well, such as 

 degree of maturity, fatigue, momentum, and structure and condition 

 of the leaf. As the structure and condition of the plant and its parts 

 also have an effect upon stomatal movement, these are considered 

 as well as the physical factors. 



LIGHT. 



The importance of light in causing stomata to open has been known 

 for many years, but the manner in which it operates is not yet 

 thoroughly understood. Lloyd (1908) has shown that in Verbena 

 ciliata, as well as in other plants, starch almost wholly disappears 

 during the early forenoon, when the stomata are at their widest, and 

 increases toward evening with the closing of the stomata. Iljin 

 (1914) found, by means of the plasmolysis method, that when the 

 stomata were open the guard-cells had a much higher osmotic pressure 

 than the surrounding cells, and attributed this to the conversion 

 of the starch. The appearance of light was considered by him to 

 initiate enzymatic change of starch into sugar. The greater concen- 



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