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106 



THE CAUSES OF FLUCTUATIONS IN TUKGE3CENCE 



the dep 



of the primary petiole in consequence of 



dent rapid di 



distal leverage which is present* 

 It has already been pointed out that very brief exposure to the direct rays of the 



sun is sufficient to give ris 

 which, under certain cond 



slow, progressive movements of 



of the pinnules 



f soil and atmosph 



umption of the nocturnal position; but in cer 

 much accelerated that the progress of elevat 



and 



may ultimately lead to a full 

 the rate of movement may be 

 onvergence becomes visible to 



the unaided eye, or even as 



pid 



as it 



cases where the leaves 



posed 



the influence of any of the common so-called stimuli. When leaves are gently detached 



axis, elevation of the pinnules and a certain amount of convergence of the 



from the axis, 



secondary rachises usually occurs. Th 



greatly under different circumstances. 



d the rate of movement, ho 



or 



lea 



vever, varv 

 plete suppression occurring in 



connection with the presence of excessive humidity, whilst, when soil and air are 

 alike very dry, the movements of the pinnules are immediate and complete, and the 



convergence of the s 

 separation be effected 



dary rachises 



derable 



Supposing that 



the process of 



ly morning, ere the leaves have been exposed to d 



sunshine, and at a time when atmosph 



and tellur 



ditions of 



are at a 



rule all assume the maximal nocturnal position within a brief 



mean, the pinnules as a 



period, but the secondary rachises exhibit little or no appreciabl 



the leaf, having been gently laid down on a glass plate and covered with a bell gla 



t. If now 



protect 



from mechanical disturbance, be 



posed to diffused sunlight, the 



pm 



will gradually resume the diurnal position more or less completely, and will retain it for 



veiy considerable period ere slow insensible elevation sets in. 



however, be exposed to direct 



sunlight after 



Should the apparatus 



expan 



has occurred, and the bell 



be gently removed, movements very soon make their appear 



moisture are both 



the sunsh 



Where atmosph 





1 



movements are often merely of the slow, insensibly p 



veiled and comparatively feeble 



ch 



of th 



occurring in leaves attached to the 

 ditions of humidity are not so high 

 degrees of rapidity 



when exposed to direct sunshine : but when 



> 



many 



and 



S 



of the 



sunshine is stronger, they 

 rapid and complete elevat 



econdary rachises take pi 



of the pinnules 



incision of the tissues, 



they do after sudden concussion, 



Where movements of this sudden, rapid type have occurred, the assumption °of the 



precisely the same fashion 

 pplication of strong heat 



nocturnal position is not however permanent; but, unless the 



r _ — „ v „ — x rwMMM . VH , 9 uu ^ umess. me an* De very drv, is 



succeeded by a partial recovery of the diurnal one preceding the occurrence of" final 



on the other hand, the movements 

 , continued exposure gives rise 



slow 



progr 



have been 



movements of elevation. Whe 

 ;hout of a slow, insensible charact 



interruption in their progr 



to no 



Now, these phenomena can hardly be accounted for satisfactorily on the theory that 



the movements, and, especially the rapidly executed movements 



dent on the 



of stimulation and active contraction of the motor 



of the leaves are depen 



movements m many cases present 



organs 



called stimuli, being as abrupt, as complete and 



characters of those following the action 



o 



according to 



the 



spreading as any of the 



theory, they must, therefore, be regarded as owing to the 



are 





The 

 f so* 



and. 



of stimulation attending exposure to direct sunshine 



happen that, so long as leaves retain the 



But if this be so, how does it 



normal relation to the 



axis 



> 



exposure to 



