80 THE CAUSES OP FLUCTUATIONS IN TURGESCENCE
Kig. 1). Tha mkliib is clirectod upwards and f o i w d s at an acute angle to the
line of the secondary racliis, so that the tme posterior or outer surface of the keel
faces upwards and backwards, and the anterior, or axillary surface, downwards and forwards.
'I'he movement acconipanying the transition from the dim-nal to the noctiu-nal
position is thus cridently not of the simple nature of that in the pinnules of Pithecahbiim,
but one in wliieh convergent elevation is combined with rotation as it is iu the
pinnre of Castù alapt or ihe pinnules of Umxmi ghnca. The ultimate depression of the
midrib which is so conspicuous in them is, however, absent, apparently in relation
to the absence of such a considerable aeeumulation of pulyinar parenchyma over the base
of the under smface of the outer half of the lamina, aud therefore to the upper side of
the keel of the midrib iu its rotated position, as is present there.
The phenomena of periodic movements iu Mimosa puiUcii onco more aflord a
striking example of the fact that the assumption of the diurnal position imphes a
departure from a previously pcimanent one, and assumption of the noctm-nal one a retm-n
to it. THs holds good for every part of the leaves, for, although according to ordinarily
accepted belief, the normal nocturnal posidou of the primary petioles is one of deep
depression, this is most certainly not the case in healthy plants exposed to natural Condilions
and wliioh have not been exposed to any distm-banee.
Under such cii-cumstanoes the noctm-nal position of the primary petioles is one of pure
reversion to the originally permanent one of sUght elevation (Plate V, Fig. 14), horizontality,
or slight depression; but it tUOEers from it in being a very unstable one winch
is Yei-y readily replaced by one of extreme depression. Very shght meclianieal distm-banee
is enough to secm-e displace-.ni-nt 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 nol-mal nyctitropic phenomenon in healthy
plants exposed to natural conditions, and when it is present it is owing to pi-eciscly the
same causes which give rise to its appearance during the day. The causes act with
»teat readiness because the normal nootarnal position is one of extreme instability,
connected with depression of osmotic property in the pulvinar parenchyma whicll
makes tor elevation {-ide Appendix 0), but they do not differ in nature from those
acting dim-nally in any respect. Under those circumstances it is a matter of surprise to
lind L c h 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 noctm-nal or a
diurnal one. According to him diui-nal 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 iu pjndulum fashion; wliilst in
nocturnal depression no such diminution occurs, and the position is a rigid one.* As
the position in any ease has precisely the same causation, it would be strange indeed
were any such difference present, but as a matter of fact it is not. Tlds is only an
example of various authoritative state uents regarding the phenomena presented by the
leaves of Mitma jmdka, which are most certainly absolutely incorrect as regards plants
™wn under noi-mal cü-cumstances ( Vite Appendix D). The phenomena may present
tíiemselves in the case of pot-plants with hmited root supply and exposed to the abnormal
atmospheric conditions ordinarily present in conservatorie.5, but they certainly are not
present under natural conditions, and therefore cannot be regarded as normal to the
plant. ^
• Saohs, Vorlesuu!;, XXXVII, s. ÍSíl,
IN THE MOTOlt OEGANS OE LEAt'ES. ^^
The periodic movements of the leaves of Mimom pudka are not only typically nycti.
tropic in respect of tlie nocturnal position being one of reversion to a previously permanent
one, but also iu respect of the phenomenon of the attainment of the maxhnal noctm-nal
position during the first part of the night and gradual departure from it subsequently
quite apart from any photic influence. The amount of reversion towards the diurnal
position exhibited by the pinnaj and pinnules is comparatively slight, but ele-vation of
the inimary petiole during the later hours of the night advances so far that in plants
exposed to nonnal conditions the maximal elevation for the twenty-four hours is attained
immediately before sunrise. This at first tight appears to be an anomalous phenomenon,
but in reality it may be readily accounted for. The maximal elevation which occurs before
sum-isc is not to be regarded as indicating that the inferior parenchymal tissue of the primary
pulvinns then attains it maximal timgeseence, and, consequently, its maximal strength,
f t merely indicates the presence of relative maximal strength in relation to distal
leverage. The inferior mass of pulvinar parcnehyjua has constantly not merely to contend
with tlie depressant action of the superior one, but also against the leverage of tlie
distal portions of the leaf. Both of these, arc subject to variations, and the action of tiie
leverage is so important as an agent in déteiiuining tlie exact position at any given time
that any considerable reduction of distal weight may not merely equilibrate a temporary
increase in relative strength of the superior mass of pulvinar parenchyma, but may well
go farther and secure that the entire resistance which the inferior mass has to overcome
shall actually attain a miinmum under the cii-eumstances.
The importance of distal leverage as a factor iu affecting the posnion of tlie
primai-y petiole quite apart from any alterations in the relative strength of the opposing
masses of pulvinar parenchyma, is indicated very clearly by the phenomena which follow
amputation of the pinnas of fully expanded leaves {vide Appendix A). The first effect of
this normally is to give rise to rapid and excessive depression of the petiole; but
elevation soon sets in and advances'so far that the petiole not merely rapidly regains its
original position, but becomes much more elevated or convergent to the hue of the axis,
than it was whilst the leaf was intact. The effects of alterations in distal leverage ou
the position of the primary petiole are also very conspicuous in cases iu winch the pinute,
in place of being removed, are merely exposed to conditions detei-mimng considerable
diminution in their weight. When a plant, the leaves of which are fully expanded, is
suddenly transferred to an atmosphere very much drier and considerably warmer than
that to wliieh it had previously been exposed, elevation of the pinnules begins almost
immediately and advances rapidly until they have completely assumed the uoetuinal
position. This phenonienon is clearly owing to loss in active tm-gesceiice connected with
sudden aud excessive trauspiratory loss accompanied by obstructive expansion of the
gaseous contents of the water-vascular system. But the primary petioles show no tendency
to assume the position proper to the early part of the night. They show no tendency
towards depression, but, on tlie eontraiy, gl-adually become very liighly elevated. It
cannot be imagined that this is the result of any increase in the absohite strength of the
mass of inferior pulvinar parenchyma which makes for elevarion. The tissues iu the
pulvini, whether petiolar or pinnular, must alike be subject to dhninution in turgesecnco
and consequent loss of absolute strength under the eireulnstances to which the
plant is exposed; but the loss in strength in those pulvinar tissues in the primary
pulvinus which make for elevation is more than counterbalanced by the excessive
TI.