96 THE CAUSES OP FLUCTUATIONS IN TUEGESCENCE 



the influence of hot dry air and a dry soil, the leaves of Mimosa pudica, PUhecohbium saman 

 and Cassia alata alike assume the nocturnal position, and free supply of water causes 

 them alike to revert towards the diurnal one. All the leaves which have been speciallv 

 considered in the preceding chapters alike exhibit movements under the influence of 



certain common conditions, such as exposure to or deprivation of light 



transp 



for th 



6 



a 



no 



tion, defective root-supply of water, and mechanical agitation; but they differ greatly 

 from one another in regard to the rate at which they indicate the effect which these 

 conditions produce. But mere differences in onset and rate of movements arising under 

 the influence of like common conditions cannot be taken as any satisfactory evidence that 

 they are owing to different causes, or that the functional properties of the tissues of the 

 motor organs differ from one another in kind. The phenomena afford no sufficient proof 

 of the existence of any essential difference in the functional properties of the tissues, and 

 the structural features of the motor organs are such as satisfactorily 

 differences which are present in different instances. There is, therefore, every ground fo 

 regarding the cause of movement as alike in all cases, and the differences 

 merely from the rate at which it is possible for it to produce an appreciable^ ect 

 Water will escape more rapidly from a vessel with a large orifice than from one with 

 small one, but this does not show that the cause of escape is different in the two cases. 



The structural features of the motor organs of the leaves of Mimosa pudica afford 

 satisfactory grounds for supposing that they are endowed with any peculiar functional 

 properties, and are such as to explain why redistributions of fluid may occur with excep- 

 tional rapidity within them ; so that there is no reason for the conclusion that the 

 movements which they conduct differ in their nature and origin from those which are 

 conducted by the motor organs of other nyctitropic leaves. They certainly give no 

 sufficient grounds for assuming that the movemants are in them, 

 nyctitropic leaves, determined by alterations in tnrgescence depending on active contrac- 



of the protoplasts of the tissues, in place of alterations in the osmotic properties 



n sap, alterations in the relations existing between general supply and loss of 

 water, or alterations in conditions affecting local filtration; but they do afford „ 

 satisfactory explanation of the fact that the movements should be more rapidly conducted 

 than they are m ordinary nyctitropic leaves. As, however, the belief in the essential 



movements on the exercise of active contraction by certain of the 

 protoplasts in the motor organs is so firmly and widely established, it is desirable that 

 an attempt should be made to determine how far the phenomena of movement __« 

 realy afford any support to it, and how far they are in any way inexplicable as the 



result of action of the factors which serve to determine tho L^JL f * • 



-i I w uetormine tne occurrence of movements in 



In approaching this question it must in the first nU™ K a k • • j „ . j 



b „i • A ,, .. . lLW nrsc P lace be borne in mind that, under 



any more 



of the 



a 



dependence of 



them 



normal circumstances, the tissues of Mimosa pudica 



ptionally large bulk of 



liquid under very high tension, and that thfc tension IT^ZZd 7 # T 



facilities for general translators loss and for oL rT2 2 1 T T 



within fWai™ no,f,- n i t. - • , redistributions of masses of water 



witnin certain particular areas. It is evident, thus that all +h* . jv * * ui 



eauilihnnm »™ ™™ 4. • y • , , ' a11 the conditions of unstable 



equuiDnum are present in very hi^h decree an! tKaf n, i ,. ., 



liable to undergo constant flulatfons IfZZ e ^£"f ^ ^ ""?* ? 

 relations existing h^ oan . i , V corresponding to any fluctuations in the 



that the T 7 SS u e T I *"*»»« k. of water. Haberlandt affirms 



qiua, wluch under most cn-cumstances escapes in such i^ blllk fl . om the 





