101 THE CAUSES OF FLUCTUATIONS IN TURGESCENCE 



If two plants of Mimosa pudica be exposed side by side to strong direct sunshine, 

 the one being included in a moist chamber and the other in a sulphuric acid chamber* 

 the pinnules of the former will remain nearly fully expanded, but those of the latter 

 will pass into the maximal nocturnal position. When exposed to diffuse light, on the 

 contrary, the pinnules in both cases alike (granting of course that sufficient soil moisture 



pplied to the plant whose leaves are exposed to a dessicated atmosphere), attain a 

 al degree of expansion. The displacement which occurs in the leaves in the 



is 



maximal ueg 



moist chamber under exposure to 'direct sunshine may be fairly credited to the onset 

 of a certain amount of transpiratory loss, for, as the exposure implies continous elevation 

 of temperature within the chamber, absolute atmospheric saturation cannot be present 

 within it. In so far as illumination and temperature go, the two plants are exposed to 



precisely like conditions, and yet the pinnules undergo only slight displacement in the 



case and complete displacement in the other, thus leaving the differences in 



one 



transpiratory loss as the only factor accounting for the differences in the result of 

 exposure in the two cases. 



In cases like the above, movements occur under the influence of the increased 

 transpiratory loss incident on exposure to the sun's rays, but equally considerable 

 movements attend excessive transpiration caused by other means. If a pot-plant of 

 Mimosa pudica in which the leaves are in the normal diurnal condition be transferred 

 from a relatively cool and moist atmosphere to one which is considerably warmer 

 and very dry, the pinnules gradually assume the fully developed nocturnal position, 

 although the conditions of illumination to which they are exposed remain precisely 

 as they were before. Very little, if any, perceptible displacement occurs in the 

 secondary rachises, but the primary petiole becomes extremely elevated. Here certainly 

 no stimulant action of the sun's rays can come into play as the determinant 

 of the movements, and as we find the latter manifesting themselves at temperatures 

 under which the diurnal position is fully maintained so long as the air is 

 humid, the only thing which is left to account for them is the excessive transpiratory 

 loss to which the tissues are exposed. It is, of course, possible to assume that 

 excessive transpiratory loss gives rise to active protoplasmic contraction ; but there is no 

 evidence whatever that it actually is so, and there is no question that it does give rise to 

 loss in turgescence from purely physical causes. The rate of movement of the pinnules 

 varies with the temperature of the air and, where this very considerably exceeds that of 



the atmosphere to which the plant was previously exposed, sudden rapid movements 

 may occur in a certain number of them, accompanied occasionally by sudden depression 

 of the primary petiole. But a still more remarkable phenomenon normally follows 

 the removal of the plant from the artificially warm and dry atmosphere to that 

 to which it has been exposed {vide Appendix E). If a plant be retransferred to 

 its original environment at a time when its pinnules have only partiallv assumed the 

 nocturnal position and every care being taken to avoid any mechanical agitation, no 



farther movements of elevation take place in the pinnules, but, after a brief pause, all 

 or a majority of the primary petioles become suddenly and rapidly depressed. Now, this 



sequence of events which is utterly unexplicable on the theory that the movements 



* In all cases in which exposure in a sulphuric acid chamber is mentioned, it is assumed that effectual means, 

 similar to those described on page 77, were taken to secure that the only additions to atmospheric humidity were derived 

 from transpiration. " 



