Page:Researches on Irritability of Plants.djvu/68

Rh shown in fig. 17, where the stimulus of sunlight was applied for 2 minutes followed by a period of recovery for 13 minutes.

If the stimulus applied on the upper half be strong or long continued, then the excitatory effect is transmitted across the pulvinus to the more excitable lower half. In these circumstances the 'up' is converted to 'down' response, on account of the greater contraction of the lower half of the pulvinus. Thus under any form of diffuse stimulation the resultant response in Mimosa is brought about by the differential excitabilities of the upper and the lower halves of the pulvinus. We should also bear in mind that the slight differential contraction-effect in Mimosa leaf is very much magnified by the long petiolar

index. There are, again, numerous pulvinar organs whose responsive movements have passed unnoticed. In Desmodium gyrans there are two conspicuous pulvini; the primary pulvinus is at the junction of the petiole with the stem; there is a secondary pulvinus at the junction of the petiole with the terminal leaflet. The primary pulvinus appears at first sight to be insensitive. But on attaching the primary petiole of Desmodium with the writing-lever, I obtained the series of responses under a very feeble electric shock, as seen in fig. 18. In this particular case the recovery is practically complete in 15 minutes. Other pulvini also exhibit differential contraction under diffuse stimulation. Thus the terminal leaflet of the bean plant (Vicia Fava) exhibits