but by a cUscliai-gc of liquid from the cut surfaces. In most cases the discharge is Tory
couspicuous; and, uuder circumstances in which movements do not occur very rapidly, it
can readily bo dcteniiined that it is the primary result, and that movements only ensue
at an appreciable inten-al after its occurrence has begun to manifest itself. For example,
if the tip of a pinnule bo gently amputated at a time when tollm-ic and atmospheiic
humidity is excessive, and if all contact disturbance of the tcrtiaiy pulvinus have beeu
successfully avoided, no movement whatever occm-s for a considerable interval. But
during this interval conspicuous exudation occurs on the cut surfacc, a prominent droplet
of liquid appearing on the divided extremity of the midi-ib, and smaller drops presenting
themselves near the margins of the lamina over the cut ends of veins. ]3ut wherever
active exudation of this natui-e is present, wo have to deal with conditions which imply
no mere local redistribution of liquid throughout the tissues, but an active discharge and
loss of a portion of the general stock of liquid available for the up-keep of turgescencc,
and heuco a disturbance in the pre-existent equilibrium between supply and loss. This
disturbance must necoisarily be liable to extend to a greater or less degree according to
the actual amount of loss which takes place and to the conditions of the tissues, of the
soil, and of the an: at the tijno at which it occurs. There is, therefore, good ground to
expect that such "stimulation" should bo more frequently and more extensively followed
by phenouiena of difiusion of movement beyond the site of primary origination than is
" stimulation" by agencies giving rise to mere alterations in local pressure and filtration.
Even the most casual observation is amply sufficient to show that this theoretical
; fully met by actual fact. Whilst isolated pinnular movements are, under
demand
aveiuge
isolated
conditions
atmospher
ionditions of humidity, the noi-mal
aaovements are quite exceptional phen<
, of excessive humidity, after iiicisioi
and telluric humidity are, and havi
of lamina
leuce of contact-" stimulation," pui'ely
lena, only occui-ring in association with
of the tissues. Where conditions of
for some time been, excessively liigh,
amputation of the tip lally followed by movement in the
injured pinnule only, or, in the case of heavily-laden leaves, by action in it and thi
primary pulvinus alone; but, under conditions of average humidity, and at times when
contact is followed by strictly localised movement in the primarily affected pinnule,
amputation or incision of the tip of a pinnule is followed by movements extending
along the entire course of that secondaiy rachis, at least. Where conditions of
humidity are higher, the propagation of movement becomes more limited and extends only
partially over the course of the rachises ; and, on the other hand, when conditions are such
that contact is liable to be followed by diffusion of movement, incision or amputation
is attended by movements which are often propagated tliroughout the entii-e area of the
leaf. When contact-"stimulation" is attended by strictly localised movement, incision
or amputation is ordinarily followed by movements extending over a portion of the leaf,
and when contact is attended by a limited amount of diffusion of movement, incision or
amputation is ordinarily followed by movements extending throughout the entire leaf.
The only possible explanations of these differences are either that incision is a stronger
.stimulant to protoplasmic contraction, or a more efficient determinant of alterations in the
I)re-existant relations between supply and loss of liquid than contact is. According to the
ordinarily accepted theory we must assume that it is the foi-mer vi-ithout, however, having
•any independent evidence that it actually is so ; according to the purely physical theory
of movement we are not obliged to make any groundless assumption, as the phenomena
are such as to leave no room for doubt that it actually does give rise to greater
disturbance in the relations between loss and supply of liquid than any mere
contact is capablc of determining.
There can be no question that rupture of the continuity of the tissues by incision
is a much stronger determinant of the diffusion of movement than mere contact
impulses are, but in this rcspcct it falls far short of injuries effected by means of heat.
The movements attending the application of an open flame, of highly heated metal, or
of the carefully focussed rays of the sun to individual pinnules aie never of a strictly
localised nature. Even under conditions of maximal humidity a certain amount of
diffusion of movement occurs; under circumstances -when incision is followed by movements
extending over the entire secondary rachis to which tlie primarily injured
pinnule belongs, the application of heat gives rise to movements extending over
the entire leaf, and when incision is followed by movements extending over the
entire leaf, heating is followed riot mciely by these, but by movements in a varying
but sometimes considerable number of leaves along the course of the axis to which
the prhnarily affected one is attached. This may at first sight appear as though it
were adverse to the physical theory of movement, and only explicable as the result
of the presence of differences in degree of specific protoplasmic stimulation, but it is
only to casual observation that it does so. The evidence of the occurrence of any
considerable exudative loss of liquid from the tissues is certainly not as a i-ule so
conspicuous as it is, in cases where the continuity of the tissues is intennpted by
means of incision. This is, however, in great part explicable as the result of the
different conditions to wliich any exudation is exposed in the two cases. In cases
of incision we are dealing with mere rupture in the continuity of tlie tissues, and, so
long as atmospheric humidity is not excessively low, any liquid which escapes
naturally tends to accumulate locally, and therefoi-e to appear conspicuously. But
where the tissues are injured by the application of heat, localised conditions for excessive
evaporation are necessarily present, and therefore an absence of local accumulation
of liquid does not imply an absence of local exudation. Where a considerable mass of
soft succulent tissue is present, as in the case of young axes, the local application
of heat is often followed by conspicuous indications of exudation even in spite of
the attendant excessive evaporation; but when the mass of tissue is limited in
relation to the heat, as it is in the case of pinnules, the rapid evapor.ition masks
the discharge. The rise in evaporation, moreover, is not of coni-se strictly localised
to the spot to which the healing agent is dii-ectly applied, but must extend over
a certain area beyond this, so that there is not mere local exudation and evaporation
from the actually injured tissues, but a cause for sudden increase in transpiratory loss
in neighbouring and intact ones. This in itself is sufficient to account for the fact
that heat should be a more efficient determinant of movement than incision is. In cases
of incision the primary alteration in the condition of the tissues is a strictly localised one;
whilst, in cases where heat is applied, there is not merely inti-insic local change in the
tissues in the immediate site of apphcation, but an alteration in the extrinsic conditions
to which neighbom-ing tissues are exposed. But another circumstance also comes in to
render heating a more efficient cause of extensive movement than simple rupture
of the contimiity of the tissues is. The liquid which exudes from the tissues on
incision very rapidly evaporates and condenses when exposed to the air, and soon forms
a protective stratum precluding farther escape from the divided surfaces; and as the
local injuiy to tissue is extremely limited, the site of incision ceases to be a weak