102 



THE CAUSES OF FLUCTUATIONS IN TURGESCENCE 



condition of relatively stable equilibrium ; but in the case of the primary pulvinus 

 this is discounted by the coincident increase in distal leverage, whereas in the 

 tertiary pulvini it is either not interfered with or positively strengthened. It can be 



wonder, then, that a disturbing factor should be capable of giving rise to effects 



differing in degree within the 



two areas. 



Phenomena of a like nature often manifest themselves in plants which are suddenly 

 transferred from a saturated atmosphere to one of the same temperature, but considerably 

 drier. Under exposure to the influence of an artificially saturated atmosphere in a 

 moist chamber, if sufficient illumination be provided, the pinnules and secondary 

 rachises of the leaves assume their maximal diurnal position of extreme expansion. 



But the primary petioles, in place of 



tending 



to sink as they do under natural 



diurnal conditions, become highly elevated. All parts of the leaf at the same time pass 

 into a condition of abnormally stable equilibrium, so that, as a rule, no movements 

 follow comparatively violent agitation. But, if the cover of the chamber be suddenly 

 and gently withdrawn so that the tissues are exposed to a relatively dry atmosphere, 

 movements occur after a brief interval. These, however, are frequently confined to the 

 primary petioles, which undergo rapid and deep depression, and it is only occasionally 

 that any conspicuous rapid elevation of the pinnules manifests itself. On the theory 

 that movements are essentially due to active contraction of the protoplasts of the motor 

 organs, these phenomena must be taken to indicate that transfer from a saturated to a 



* a 



relatively dry atmosphere causes protoplasmic stimulation more readily in the protoplasts 

 of the primary pulvini than in those of the tertiary ones. But on the physical theory, 

 no such arbitrary assumption is called for. When the distal portions of the leaves are 

 saturated with moisture and have been continuously exposed to conditions favouring active 

 assimilation, their weight must attain its maximal development, and with this the strain 

 upon the primary pulvinus must be proportionately increased. Even in the absence of 



saturation, 



prolonged 



exposure to conditions favouring assimilation causes sufficient 



increase in leverage to aid materially in inducing a depression of the leaves from their 

 late nocturnal position of extreme elevation; but when transpiratory loss is absent, it is 

 unable to cause depression in spite of the aid which mere excess of distal fluid must 



■ 



yield to it. The strain, however, upon the primary pulvinus in 



must be veiy great, and when the plant is suddenly exposed to conditions 



maintaining 



erection 



plyin 





the 



an abrupt onset of transpiratory loss disturbing the pre-existent condition of liquid equili- 

 brium, the effect naturally manifests itself most readily in it as the weakest part in 

 motor apparatus. Where the atmospheric conditions are such as to give rise to trans- 

 piratory loss in limited degree only, the disturbance tells only on the weakest part of the 

 apparatus, and depression of the primary petioles occurs; where the loss is greater, 

 movements occur in the pinnules, but the loss must be very excessive to induce any 

 appreciable movements of convergence in the pinnae, because in the secondary pulvini 



conditions favouring a rapid loss in turgescence 

 they are in the other motor organs. 



are present in much lower degree than 



t 



T 



directly to the p 



extent, rapidity and propagation of movements are then unequivocally related 



of conditions in the environment 



loss of water. But if disturbances of the ( 

 essential factor which determines all movements 

 cence connected with fluctuations in solar stimu 



regulating 



general 



ppty 



an 



d 



quilibrium between supply and Joss be 



dependent on alterations 

 and assimilatory activity 



in turjres- 



& 



niov 



