]22 THE CATTSES OP FLUCTUATIONS IN TUEGESCENCE
all, muecessaiy for the occurrcnce of jn-opagathat
tho presence of such a system is specifically
that the presence of sieve-tubes at s
tiou of movement, and yet maintair
related to the phenomenon.
Tho results which may be obtained vhere ai-eas in the course of entire axes in
place of petioles are killed by means of tho locul application of ]ieat are even more
convincing. Areas of a quarter of an inch in length in the course of axes may bo
readCy killed by boiling, or more conveniently by thoroughly scaring them with heated
forceps. It is easy to kill several successive areas at intervals along the course of a single
axis, and yet propagation of movement will take placc from points on the proximal side
of tho lowest dead ai-ea to points situatea on the distal side of the highest one. For
example, taking a small plant and searing the axis in two points so that one or more
leaves are situated below the lowest scared area, one leaf between the two seared areas,
and several beyond the distal seared area, it will be found that, when external conditions
are such as to favour the occui-rence of propagation of movements, tho application of
heat to the base of tbe axis is followed b y action in all the leaves alike, ^Yhether beneath,
between, or beyond the seared areas {vide i^ppendix G). It is easy to detei-mine in
such cases, by means of sections carried through the entire thickness of the axis, that
the tissues have been completely killed throughout (Plate Y, Fig 7). This, of com-se,
demonstrates that any continuity of living protoplasm is unnecessary, and on Haberlandt's
theory forces us to assume that the initial impulses originatiog in his specific conducting
system undergo a primaiy transfer to the wood, a retnrn to the specific conducting
system, a renewed transfer to the wood, and a renewed transfer to the specific conducting
system, ere they tell on any leaves situated beyond the distal seared area. Sm-ely
the specificity of the Rcizleitmde system" is, inider such circumstances, hard to recognise.
But it is not only by such experiments that the groundlessness of liaherlandt's
assumplion may be demonstrated. The phenomena, which have been already described
as following the local application of heat to points in matiu-e, hard axes are equally
conclusively opposed to it. In such cases, if the diffusion of movement were essentially
dependent on the propagation of fluctuations in pressure aloug a system of tubes filled
with liquid, it ought assui-edly to take place in both directions ahke. But this it most
certainly does not do; for, whilst granting that external conditions are favourable to the
occurrence of movements, centrifugal diffusion normally occurs with the greatest certainty
and to an indefinite extent, centripetal diffusion is normally entirely absent, and, in tlie
exceptional cases in which it does occur, is entirely limited to the very immediate neighbourhood
of the site at which heat is applied. In any continuous system of tubes filled
with liquid, the local apphcation of heat to any particular point in its course must
inevitably give rise to an increase in pressure which must be propagated in both directions
alike ; and, if increase in pressure throughout the system were an essential factor in
determining the occurrence of movements, movements would certainly manifest tJiemselves
in both directions, alike. But as a matter of fact, the rise in pressure is propagated in
both directions, whilst movements occur only in one.
Haberlandt is unquestionably riglit in maintaining that much of the liquid which
exudes from the tissues of Mimosa puclica on incision is derived ¿•om the highly turgid
tissues of the soft bast, but tbe latter do not hold any truly specific relation to the
occurrence of propagation of movement. They provide an apparatus facilitating the
sudden escape of liquid fi-om the tissues, and hence an apparatus facilitating sudden
IN THE MOTOR ORGANS OF LISAVES. 19.1
fluctuations in the
of propagatic
Kquid contents of the latter, but here their relation to the occurrence
if movements ccases. Their excessive turgidity and their structure
render them effective agents in securing the loss of large quantities of liquid on incision,
and this provides for a disturbance in the pre-esistent equilibrium between supply
and loss of liquid generally. Their excessive turgidity secm'cs a large discharge of
Hquid. and their high osmotic capacity, on which their normal turgescence depends, must
render them a source of excessive di-ain upon tho generally available stock of liquid. Any
sudden discharge of a large quantity of liquid from them implies a suddenly increased
demand upon the general water-supply, and, owing to their excessive osmotic capacity,
they are well adapted to worst other activG tissues in tile struggle which must ensue until
a condition of liquid equilibrium be onco more established. As has been already pointed
out, the presence of any extensive system of excessively osmotic elements is in itself an
element faTOUi-ing instability of equihbrium on the relations between general loss and
supply of liquid in less highly osmotic tissues. When any great loss in tm'gidity occurs
•withia it, apart from any alteration in its functional activities, and consequently of its
osmotic capacity, tho lafter co-operates with active transph-atory loss in giving rise to a
great draught upon the geireral stock of flttid available for the maintenance of turgescence,
whilst tho coincident fall of pressm-e probably induces additional filtrative escape from'
other active tissues. Tbe " :Beizleitaiâe systim" is an apparatus through means of which
the conditions of equilibrium between general supply and loss of liquid may be very
readily disturbed and, therefore, is an apparatus through means of which movements ]
readily be indircctly determined; but is not pecuhar or specific in this respoct for,3 may
we have ah-eady seen, shnilar effccts may be produced by means of changes takint^
place within the wood. ®
Haberlandt affirms that any iluctuations of ¡n-essure within the system of seive-tubes
whether of a positive or negative natm-e, are capable of acting as stimidants to tho contractile
tissue of the motor organs. But the phenomena attending tho local apphcation of
iieat to mature, hard axes show that fluotnalions of pressure witluu it are not necessarily
elScicnt factors in determining movements, and also that it is only negative iuotuations
in general liquid pressm-e that arc anywhere efficient. In such cases we have positive
fluctuations in liquid pressm-e arising within the sieve-tubes in both directions alike, and
movements occm-ring only centrifugally, that is in the direction of diminished liquidpressure,
within tho water-conducting system. Negative fluclualions in Hquid-pressm-e in
tliese cases occur in all distally siiuatod areas in consequence of the obstructive expansion
of the gases of tho water-conducting system! in cases of incision, on the other hand,
similar ^fluctualions arise both centripelally and centrifugally owing to tho sudden escape'
of considerable masses of liquid from tho tissues. In the first instance, movements occur
only oontrifagolly, in the second both centripetally and centrifugally ; in other words, in both
instances alike, the distribution of movomcnts coincides with the distribution of negative
tluetuaticns in liquid-pressure. Such fluctuations may arise in various ways; they may
arise in conscquence of escape of liquid from the system of sieve-tubes, of obstmctions
arising in the water-conducting system of tho wood, of sudden .contaction in the
gaseotts contents of the latter, of increased trauspu-atory loss, or of sudden arrest of
root-supply connected with separation from the axis; and in all cases alike they are
liable to he followed by the occurrence of spreading movements. Negative fluctuations
1 liquid-pressure are the specific determinants of all spreading movements which