THE CiTJSES OP PLUCTniTIOTS IK TUEGESCESCE
H tn-0 plants oi Mhmt, ,nulm bo exposed side by side to strong dirocf stinslnno
the one bomg mdnded in a moist oh.iinber and the other in a snljAaric aoid chamber*
the pmnnles of tlio former ivill remain noai-ly fully expanded, but those of tho latter
will pass into the maximal noctm-nal position. When exposed to diflaso light, on tho
contrary, tbo pinoulos in both cases aKke (gi-anting of conrse that sufficient » i l moisture
)S snpplred to the plant whose leaTes are exposed to a dessicated atmosphere), attain a
maxiiual degree of expansion. The displacement which occm-s in the leaves in tho
moist cbamber under exposure to direct sunshine may bo faMy credited to the onset
of a certain amount of t:-anspiratory loss, for, as the exposure implies continons elevation
of temperattire within the chambor, absolute atmospheric saturation cannot bo present
within it. In so far as illumination »nd tompcraturo go, tho two plants are exposed to
precisely like conditions, and yet the pinnules undergo only slight displacement in the
one case and complete displacement in tho other, thus leaTing the differeirces in
transpiratory loss as the only factor accounting for tho differences in the result of
exposure in the two cases.
In cases like the above, movements occur under the influence of the increased
transpiratory loss incident on exposure to the sun's rays, but equally considerable
movements attend excessive transph-atioii catiscd by other moans. If a pot-plant of
JJimm pudiai in which the leaves are in the normal diurnal condition bo transferred
from a relatively cool and moist atmosphere to one which is considerably warmer
and very dry, the pinmdes gradually assume the fully developod nocturnal position,
although the conditions of illumination to which they aro exposed remain precisely
as they were before, Veiy little, if any, perceptible displacement occurs in the
secondary i-achisos, but the primary petiole becomes extremely elevated. Here certainly
no stimulant action of tho sun's rays can come into play as the detorminont
of the movements, and as we find the latter manifesting themselves ot temperatures
tinder which the diurnal position is fully mointained so long as tho air is
humid, the only thing which is left to acccant for them is the excessive transpiratory
loss to which tho tissues are exposed. It is, of coarse, possible to assume that
excessive transpiratory loss gives rise to active protoplasmic cimtraction ; but there is no
evidence whatever thot it actually is so, and there is no question thot it does give rise to
loss in turgescence from purely physical causes. The rate of movement of the pinnules
varies with the temperature of the air and, whore this very considerobly oxceeds that of
the atmosphere to which the plant was previously exposed, sudden rapid mevements
may occur in a certain number of them, accompanied occasionally by sudden depression
of the primary petiole, lint a still more remarkable phenomenon normally follows
the removal of the plant from the artificially warm and dry atmosphere to that
to which it has been exposed (¡.¡¿e Appendix E). If a plant be rotransferred to
its original envir-onment at a time when its pinmdes havo only partiallv assumed the
nocturnal position and every care being taken to avoid any mochanicaf agitation, no
f a r t h e r movements of elevation take place in the pinmdes, but, after a brief pau.so, all
or a majority of the primary petioles become suddenly and rapidly depressed. Now, this
is a sequence of events which is utterly nnexplicablc on the theory that the movements
similar to tlioi
from transpira
• n siilphai
e taken M S
acid chamber is Teeniioned, it !s assumed fliat elFcctnal means,
ire that the oii\y additions lo atmospherie hnmiditj were derived
IN THE MOTOR OEGANS OF LEAVES. 105
are essentially dependent on active stimulation and contraction of the protoplasts of the
motor organs, unless indeed on the groundless assumption that exposure to hot and dry air
acts as a specific stimalant of the protoplasts of the motor organs of the tertiary pulvini,
and exposure to cool, moist air as a specific stimulant to those of the primary ones. JJut
if we accept the theory that the movemeirts are dependent on conditions affecting supply
and loss of fluid, the sequence admits of ready explanation. Sudden transfer to a
relatively warm and dry atmosphere must give rise to increased transpiratory loss and
must establish a drain upon all the turgescent elements of the tissues to which they
were not previoaly exposed, so that, unless root-supply sustains an equivalent increase, a
general loss of turgescence must tend to be established. The actual loss in turgcscence
which will occur in individual masses of tissue will be rapid and extensive in direct
relation to the facilities which they provide for the escape of liquid from the interior
of the cell-clerrrents of which they are composed. But the tissue-elements in those
masses of tissue which make for the diurnal position in tho primary and tertiar-v
pulvini afford much greater facilities for the escape of liquid, than those which make
for the nocturnal position. Hence a relative weakening of the forancr must occur, and
with this, if no interfering factor come in to modify the result, movements towards the
nocturnal position must necessarily ensue. In the case of the tertiary pulvini, nothing
comes in to interfere with the execution of such movements, and they are accordingly
carried out at rates varying with the rapidity at which the weakening loss in turgescence
occurs within the palvinar tissues raoking for the diurnal position. But, in the primary
pulvini, tho local weakening is accompanied by coincident rapid decrease in distal
leverage which discounts and more than discounts it. The mass of tissue in the primary
pulvhms which makes for elevation of tho petiole is weakened in so far as respects its
local pnlvinar opponent, but is strengthened in respect to the total resistance which it
has to overcome, and hence elevation and not depression occurs. But when tho plant
is once more suddenly transferred to its originally cool and moist envnonment, a sudden
check is put on the activity of transpiratory loss, and with this no farther elevation of
the pinnules occurs, and distal leverage, in place of undergoing any farther decrease,
begms to increase. Simultaneously another important factor oflocting the general distnbutien
of iluids throughout the tissues comes into play. The rapid cooling following
transfer to the cool air necessai-ily implies rapid contraction in all the gaseous contents
of the water-conducting, tissues, and this must tend to give rise to a back draught on
tile liquid contained within the actively turgescent ones and, therefore, to enhance
iiltrative loss from them. Ordinarily the effect of this on the pinnules is done awoy
with by the couicident cessation of active transpiration, and, consequently as a rule, no
appreciable movement occurs in them, but in the case of the primary pulvimis nothing
comes in to interfere with its action. The cessation of transpiratory loss does not here
tell against the occurrence of movement, but directly in its favour iir consequence of the
increase in distal leverage which it necessarily involves, and, consequently, depression of
the primary petioles naturally tends to occur. As will be presently demonstrated, a
sudden expansion of the gaseous contents of the water-conducting tissues may also
serve to occasion sudden movements, apparently as the result of the obstruction to
onward flow of water which it induccs if pushed beyond certain limits; but any
obstruction which arises in cases like the above on the introduction of the plant imo
the heated atmosphere is, unless it bo jn-esent in very high degree, unable to determine