t h e C.1U8ES OF FLnCTUiTIOSS I S TOEGESCESCE
the depression oE the primary petiole in consequence of the coincident ™id diminution
in dista! leverage wliich is present.
It has already been pointed out that very brief exposure to the direct rays of the
sun IS suBicieat to givo rise to slow, progressive moTemonts of eleration of the pinnules,
M'liich, under certain conditions of soil and atmosphoi-e, may ultimately lead to a full
assumption of the nocturnal position; but in certain cases the rate of movement may be
so much accelerated that the progi-ess of elevation and convergence becomes visible to
the unaided eye, or oven as rapid as it is in cases where the leaves arc exposed to
the mSuence of any of the common so-called stimuli. When leaves are gently detached
from the axis, elevation of the pinnules and a certain amount of convergence of the
secondary rachises usually occurs. The extent and the rate of movement, however vary
greatly under different cii-cumstances, more or less complete 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 secondary rachises considerable. Supposiig that the process of
separation be effected in tile early morning, ere the leaves have been exposed to direct
suiisliine, and at a time when atmospheric and telluric conditions of moisture are at a
mean, the pinnules as a rule all assume the maximal noctm-nal position within a brief
period, bat the secondary rachises exhibit little or no appreciable movement. If now
the leaf, having been gently laid down on a glass plate and covered with a boll glass to
protect it from mechanical disturbance, be exposed to diffused sunlight, tho pinnules
will gradually resame the diurnal position more or less completely, and will retain it for
a veiy considerable period ere slow insensible elevation sets in. Should tho appamtus,
however, bo exposed to direct sunlight after expansion has occurred, and tlie bell glass
be gently removed, movements very soon make their appearance. Where atmospheric and
tellnric moisture are both excessive, or the sunshine is veiled and comparatively feeble
the movements are often merely of the slow, insensibly progressive character of tbos^
occurring m loaves attached to the axis when exposed to direct sunshine; but when conditions
of humidity are not so high, or the sanshme is stronger, they occur in increasing
degrees of rapidity until, in many cases, rapid and complete elevation of tho pinnules
and coiivergenco of the secondary rachises take place in precisely the same fasliion as
they do after sudden concussion, incision of tho tissues, or application of strong heat.
"Where movements of this sudden, rapid typo have occurred, the assumption of the
nocturnal position is not however permanent: but, unless the an be very dry, is
succeeded by a partial recovery of the diurnal one preceding the occurrence of final
slow progressive movements of elevation. Where, on the other hand, the movements
have been throughout of a slow, insensible character, continued i
interruption in their progress.
I exposure gives rise to no
New, these phenomena can hardly be accounted for satisfactorily on the theory that
the movements, and, especially the rapidly executed movements, of the leaves arc depen- •
dent on the occmTcnce of stimulation and active contraction of the motor organs. The
movements in many cases present all tho chaj I'acters of tlioso follo'wiTig the action of so*
called stimuli, being as abiiipt, as complete and as spreading as any of these are, and,
according to the theory, they must, therefore, be regarded as owing to the occurrence
of stimulation attending exposure to direct sunshme. But if this be so, how does it
happen that, so long as leaves retain their normal relation to the axis, exposure to
IN THE MOTOR OEGANS OP LEAVES. 107
direct sunshine normally occasions only slow insensibly progressive movements. Why
should exposure to the sun's rays only give rise to movements which, judging from
the parallel phenomena occurring in non-motile leaves under similar circumstanccs, are
unequivocally dependent purely on increased transpii-atory loss, so long as the leaves
retain their normal connection with the axis, and to movements dependent on active
protoplasmic contraction when they no longer do so? This, moreover, is not the only
problem which has to be accounted for; for, if exposure to the sun's rays primarily
occasions active conti-actÎon of the protoplasts of the motor organs, why sliould it cease
to do so ? why sliould continued exposure be accouipaDied by partial recovery of the
diurnal position? Those arc questions which the theoiy apparently can answer only
by means of a series of arbitrary assumptions.
But if this theojy be incapable of accounting for the phenomena, can that which
ascribes the occurrence of rapid movements to pm-ely physical causes do so any more
satisfactorily ? When a leaf is suddenly detached from the axis, an abrupt arrest
is of course in any case put to any fui-ther root-supply of water. But in Mùwsa j>udicu,
owing to the high liquid tension throughout the tissues, this is accompanied by active
exudative discharge of a relatively large mass of liquid from the extremity of that
portion of the petiole which remains attached to the distal portion of the leaf. The
liquid which escapes may be mainly derived from particular tissues or it may not; but in
any case, the effect must be a general loss in liquid tension throughout the entire leaf,
and consequently a tendency to increased drain upon the active tissues. In eases where
the tissues are excessively saturated as the result of antecedent conditions of atmospJieric
and telluiic humidity, and whi îre at the time of separation from the axis transpiratory loss
is very low, the intrinsic water-conductiug tissues contain an amount of fluid more or
less completely sufficient to make good the actual discharge of liquid attending separation,
and, for the time being, the ai-rest in root-supply, and hence either no movements at
all occur, or any which do occur arc slight and partial. With any diminution in the
amount of store-water, or any increase in the activity of transpiratory loss, the conditions
providing for the occurrence of movement will be present in progressively
increasing degree, and a point must eventually be reached at which separation from tho
axis implies ionmediate active di'ain upon the turgid elements of the motor organs.
But this drain will naturally tell most heavily and rapidly on those masses of tissue
presenting the greatest filtrative facilities and, therefore, on those which make for the
diurnal position, and hence movements corresponding to a weakening of these will occur.
The masses of tissue in the tertiary pulvini which make for the dim-nal position of
the pinnules afford great structm'al facilities for rapid filtration; boih from their inherent
delicacy and from the great excess of structural strength in their opponents, and hence
the pinnules tend to pass on rapidly into the nocturnal position. In the secondary
])ulvini the opposing masses of tissue do not present nearly such considerable differences,
and hence movements in the secondary rachises are often almost or entii-ely absent
•when the pinnules exhibit conspicuous movement, and under normal circumstances they
never exhibit tlie same activity as those of the pinnules do. If transpiratory loss be very
active, any liquid whicli escapes from the turgid pulvinar tissues on the sudden disturbance
of equihbrium of general fluid pressure on separation from the axis and any store
o£ water within tho water-conductiug tissues will be rapidly removed and the nocturnal
position of tho pinnules -n'ill remain permanent, and hence no phenomena of recovery
of the diurnal position manifest themselves in leaves which are detached from tlie axis
AK.N. HOY. BOT. GAUD, Oii.cuTiA VOL. YI,