THE CAF3ES OF FLtJCTUATIO^fS IN TUEGESCENCE
point In respect to relations between snpply nnd loss of water. The abnormal drain
on ^ flio general stoot of Ktinid witliin the tismes is purely tranporaiy in cases of
rncision. Bnt this is no longer tlie case where portions of tissue are killed by applica^
tion of heat. Any ordinary application of lieat to a partionlar point iinplios the death
of considerable masses of tissue and, as we have already seen, any dead mass of
tissue represents a permanent site of abnormally excessire drain upon the general stock
of lionid. The details which were given in Chapter II regarding the phenomena
occurring m connection with the death of portions of ares or i-achises under the influence
of the Tapour of ehloroform and heat are sufficient to show that this is the case- and the
fact is of course oonslantly brought before US b y tho difficulty which is encomtcred in
keeping portions of a leaf or shoot turgid beyond any point in which the tissues have
been killed by means which do not in any way interfere with the efficiency of tho
watrr-condncting system, whenever external conditions are sueh as to favour active
evaporative loss. In cases of the application of heat, then, we do not merely determine
temporary exudative and evapomtive loss in the site of application and its ir,mediate
neighbourhood; but we also almost inevitably establish a permanent abnormal drain
upon tho water-supply generally, and with this the liability to ditfusion ef movements
naturally increases.
In cases of contact-" stunulati<
iduce mere local redistributions of liquid
within the tissues; in eases of inei
tho general supply of liquid; in
addition we establish temporary increase in tmnspiratory loss and
abnoi-mal drain. The tendency to diffusion of movement
we give rise to temporary exudative loss from
not only this, but in
site of persistent
¡niniimiin in cases
of heatiu!
of contact; in eases of incision it increases; in eases of heating it rises to a maximum.
The two sets ef facts run parallel with one another, and" the former of then
affords an explanation of the latter, if we accept the view that the movements arc
dependent on fluctuations in the relations existing between supply and loss of liquid.
If, on the other hand, we insist on adhering to the popular theory, we must be content
to ae.sume that contact, incision, and heat are stimuli which chfl'er from one another
in then- capacity for inducing active protoplasmie contraction.
C H A P T E R X I I.
•iElic raiistE of tlic p t o p a g i i t i o n oi inoDEintiits i n ¿ H i i n a s i t )jiiïiic;i.
Whilst the moTeinents of the various paits of the loaves of Mima pulica have
by common consent been ascribed to the action of certain special masses of irritable
and contractile tissue in the motor organs, several distinct theories have been advancod
to explain the phenomenon of the diffusion of movements beyond the site in which
they are originally induced, or, according to the ordhiarily accepted theory, to explain the
diffusion of "stimulation" from one part to another. According to the older observers
Uutrochet, Sachs, Hoffmeister and Pfefier, fluctuation» in the pressure of the currcnt
of Kqnid traversing the wood played the part of moehanioal stimuli to the irritable
and contmctile elements in tho tissue of the motor organs. At a later period, and when
attention was first attracted to the fact that, as a general rule, the protoplasts of vegetable
tissues are not isolated, but are connected with one another so as to form a contniuous
reticulum, an attempt was made to substitute a nervous for a mechanical theor3-
I-V THE MOTOR ORGANS OP LEAVES, 121
Of propagation. Finally, Haberlandt has reverted to the older view ivith the inodiCcation
that, according to him, the stimulant fluctuations in liquid pressure do net normally
traverse the wood, but the system of large sieve-tubes in the soft bast which ho regards
as constitntmg a specific "RekkiteMi susUvt." His theory agrees with that of the older
writers in regarding fluctuations in liqidd pressure as serving as mechanical stimuli to
tho specific contractile apparatus in the motor organs, but afiinns that such .stimulant
fluctuations are practically solely diffused by means of the sieve-tubes of the soft bast.
He does not question the existence of special irritable and contractilo tissues in tho
motor organs, but merely modifies the older view in regard to the precise paths followed
by the agencies which are assumed to call those into activity. His theory is mamly
founded on the fact that he was able to demonstrate that the liquid which exudes from
the tissues when they are mcised is not mere water, as it should be if derived solely
from the wood, bnt is m great part composed of matei-ials which are evidently derived
from tho sieve-tubes of the bast. No one can deny that ho has successfully dcmonslratcd
tiiis, and has added greatly to our knowledge of tho histological details of llmom jjudka,
but he has certainly not succeeded in proving that his " RdzleUeaile s^Heiit'' is spccifi.
cally and essentially related to the phenomena of propagation of movement. His own
experimentai residts indeed force him to allow that propagation may occur in sjiite of
the entire absence of his conducting system throughout considerable areas ef tissue, but
notwithstanding this, liis dosii-e to establish the presence of a specific mechanical neiwous
apparatus loads hin to adhere to his theory. When he found that propagation extended
beyond areas wdiich had been entirely deunded of his " RehUitcnde systtm^ and is forced
to allow that in sueh cases difitnsion of impulses must have taken place through tho wood,
it might have been e-xpected that his views as to tho specific value of the system of sievetubes
would have undergone some modification; but apparently they were too firmly
subjectively estaljlislied at the time at 'wliicli lio made tlio obsei'vatiou to be sliaken by
auy facts which couflicted witli them. He had, of course (although ho does not appear
to recognise it) equally dearly demonstrated the fact that projjagation could occur
apart from tho presence of any continuous system of turgid sieve-tubes, in the
experiments on which lie founds his decisive rejection of the theory of propagation
by means of contiiniity of living protoplasts. In these exijcriments he killed
the tissues of a certain area in tho com-se of the petioles of leaves by means
of boiling them, and then determined the fact that "stimulation" ajjplied at one side
of the dead area was followed by the occurrence of movements on the other side.
Now this, no doubt, very satisfactorily demonstrated that diffusion of ]uovement could
not be essentially dependent on the propagation of impulses along continuous tracts of
living protoplasm, but it just as effectually proved that it was not necessarily owing to
any propagation of impulses along the course of continuous system of turgid sieve-tubes.
TJie boiling of the pctiolar tissue secured tho interposition of an area of dead protoplasm,
but it just as certainly secured the interposition of an area devoid of turgidity, and in
addition to this of an ai-ca 'within which an abnormal leakage of water was present.
Ho was evidently aware of tho latter fact, as he specially alludes to the necessity which
arises in such cases of taking special means to prevent the drying up of the parts
beyond tho boiled area, but, in spite of this, he appears to assume that the turgidity
of tho sieve-tubes remained intact. The position is an incomprehensible one; he
demorstvates that the presence of a contijiuous system of turgid sieve-tubes, and even
. EOT. Bor. Gari I YOL. YI.