80 



THE CAUSES OF FLUCTUATIONS IN TURGESCENCE 



Fig, 1). 



The midrib is directed upward 



s 



- 



and forwards at an 



acute angle 



to the 



line of the seco 



faces up\ 



ndary rachis, so that the true posterior or 



outer surface of the keel 



yards and backwards, and the anterior, or axillary surface, downwards and for- 



the transition from the diurnal to the nocturnal 



ards. The movement accompanyin 



o* 





position is thus evidently not of the simple nature of that in the pinnules of Pithecolo- 

 bium, but one in which convergent elevation is combined with rotation as it is m the 



pinna? 



midrib which is so conspicuous in 



of Cassia alaU or the pinnules of Leucwna glauca. The ultimate depression of the 



them is, however, absent, apparently in relation 



to the absence of such a considerable accumulation of pulvinar parenchyma over the base 

 of the under surface of the outer half of the lamina, and therefore to the upper side of 

 the keel of the midrib in its rotated position, as is present there. 



The phenomena of perio 



die movements in Mimosa pudica once more afford a 



striking 



rxample of the fact that the assumption of the diurnal position implies a 



a return 



departure from a previously permanent one, and assumption of the nocturnal one 



to it. This holds good for eveiy part of the leaves, for, although according to ordinarily 



accepted belief, the normal nocturnal position of the primary petioles is one of deep 



depression, this is most certainly not the 



case 



in healthy plants exposed to natural condi- 



tions and which have not been exposed to any disturbance. 



Under such circumstances the nocturnal position of the primary petioles is one of pure 

 reversion to the originally permanent one of slight elevation (Plate V, Fig. 14), hori- 

 zontally, or slight depression ; but it differs from it in being a very unstable one which 

 is very readily replaced by one of extreme depression. Very slight mechanical disturbance 

 is enough to secure displacement from it, and it is, therefore, only on still evenings that 

 the leaves retain their normal position as determined solely by absence of solar stimulation. 

 Deep depression of the primary petioles is not a normal nyctitropic phenomenon in healthy 

 plants exposed to natural conditions, and when it is present it is owing to precisely the 



same causes which give 



rise 



to its appearance 



during 



the day. The causes act with 



great readiness because the normal nocturnal posi'ion is one of extreme instability, 

 connected with depression of osmotic property in the pulvinar parenchyma which 

 makes for elevation (>ide Appendix C), but they do not differ in nature from those 

 acting diurnally in any respect. Under these circumstances it is a matter of surprise to 

 find such a skilled observer as Sachs asserting that there is a difference in the condition 

 of the pulvinar tissues according as the position of deep depression is a nocturnal or a 

 diurnal one. According to him diurnal depression following the incidence of mechanical 



impulses or other so-called stimulant agencies is associated with diminished turgescence of 



the pulvinar tissues, so that the leaf 



swings 



freely in pendulum fashion; whilst in 



■ • 



nocturnal depression no such diminution occurs, and th 

 the position in any case has precisely the same 





position is a 



rigid one. 



* As 



causation, it would be strange in 



deed 



were any such difference present," but as a matter of fact it is not. 



This 



is 



only 



an 



example of various authoritative state nents regarding the phenomena presented by the 

 leaves of Mimosa pudiia, which are most certainly absolutely incorrect as regards plants 

 grown under normal circumstances (Vide Appendix L>). The phenomena may present 



case of pot-plants with limited root supply and exposed to the 'abnormal 



themselves in tl 



are not 



atmospheric conditions ordinarily present in conservatories, but they certainly 



present under natural conditions, and therefore cannot be regarded as noimal to the 



plant. 



— «. 



* Sachs, Vorlesunu, XXXVII, i. 78V. 

























