Page:Researches on Irritability of Plants.djvu/109

86 exhibit the after-effect on the removal of the gas. It will thus be understood how important it is to maintain the external conditions constant for so long a period as two hours. The method of maintaining the testing stimulus constant has already been explained. With special care the temperature of the plant-chamber can also be kept uniform. The other factor which is liable to variation is the intensity of light. I have often noticed a fluctuation

in the uniformity of responses which was traceable to a passing cloud. I soon found that a sudden change in the intensity of light induces a marked variation of motile excitability in Mimosa. Thus on bringing a highly sensitive plant to a dark room its excitability is found to disappear. This abolition of excitability is generally speaking temporary, since the plant often regains its sensitiveness after about an hour, though still kept in the dark. The fact that under normal conditions it is the sudden diminuntion of light rather than darkness that induces depression