n THE CADSES OF PLUCTUATIONS IK TOIiGESCENCE
affecting tho presence or absence of solar stimulation have constantly been regartlod as
of an entirely pcealiar and specific natare. Tho ordinary pciiodic movementr are reeogmscd
as being connected with alterations in turgescenco dependent on ti.e presence
or absence of solar stimulation; bnt it has oppai-ently never been questioned that the
other class of movements are OTving to a certain inherent and peculiar ii-ritahilily and
contractility in the protoplasts of certain areas in the motor organs, -ivhidi enable them
to eflect alterations in tui-geseeiioe, when tlicy are called into activity by various
stinialant ageucies which are quite incapable of producing any shnilar effect on the
protoplasts of the motor organs of common nyctitropic leaves.
Tho motor organs of Mimosa fmdica aro, in fact, commonly regarded as being provided
with a sort of undifferentiated miisoular apparatus, and the faith in the esistenco
of this has been so implicit that within recent years an attempt was even miule to
ascribe the phenomenon of propagation of movement fi-om one part of the plant to
another, to the presence of an undifferentiated nervous system provided by tho continuity
of the^ protoplasts of the various tissue-dements.» This latter theory hardly merits
discussion, as it was very soon demonstrated that protoplasmic continuity is a phenomenon
of almost universal occurrence in vegetable tissues, and that the phenomena of
propagation of movement in Mmosa piuliea are in many instances qaite inexplicable as
the result of any changes occun-mg along the course of any continuous tracts of living
protoplasm. Tho theory affords, however, an evidonee of the fi.-;ed belief in the presence
of certain specific fanetional peculiarities in the motor organs of the plant, and its
abandonment only led to a renewed search for other possible agencies which might
be regarded as capable of calhng these into play.f °
The belief in the presence of certain specific in-itable and contractile appaiatas
within the motor organs of JUimom pudica appears to have arisen mainly in conseqacnce
of tlie exceptional ease with which movements may, under certain circumstances, be
induced, the exceptional rapidity with which they may be carried oat, and the extent
to which they may be propagated from one part of the plant to another; for, as has
already been pointed out, their mere magnitude is by no means exceptional, and they
agree with those which are conducted by ordinary nyctitropic leaves in leading, as a rale,
to the resumption of a position which, at an earlier period in the history of the
leaves, was tho permanent one. In addition to this, it has to be borne in mind that
the investigations in regard to the natm-e and origin of the movements have in greater
part been can'icd out in plants exposed to annatm-al conditions, and consequently liable to
exhibit phenomena of more or less abnormal character and limitation, and that they arc
consequently very imperfectly representative of those proper to the plant when exposed to
its normal environment. The phenomena which are ordinarily described as characterising
plants cultivated in pots in European conservatoiies and laboratories, as has been already
pointed out in connection with tho question of the initial noctm-nal position of the primary
petioles, certainly do not correspond in detail to those oeearring in plants grown under
natural conditions, Tho ai-tifieially-grown plants are subjcct to the influence of
limited root-supply and to abnormal conditions of atmosphere, and it is, therefore, no
wonder that the phenomena wliieh they exhibit fail to afford an exact index to those
r.sity Press, 1
t Das Beii
llio Phjaiology of Plants. By Sydaay Howard Vines, M,A., D.Sc., F.R.S. Cambridge, at tlic Oai
[0 Goircbesystem der Sinupilanze. Von Dr. G. Habcrlandi. Lcip,
IN THE MOTOR ORGAXS OF LEAVE.?. 95
oceumng in plants gi-owing under drcumstanccs pennitling of indeflnite root-extension
and impljing exposure to aU possible variations in degree of atmospheric humidity.
Moreover, rt is as a rule only in the tropics that phenomena of foliar movement occur
with sufficient frequency, and on a sufficiently large scale, to afl'ord satisfaetoiy
opportunities for the comparative study of the peculiarities of movement in many
diflcrent speeics-; and it is only by means of such comparison that it is possible to
determine how far any -phonomena occurring in individual eases are really of peculiar
and specific nature.
The movements which are executed by tho leai-cs of Mmom puiim are exceptional
and pecaliar as compai-ed with any oeeurring among nyctitropic leaves in temperate
regions; but in any region in which Neftunia. okraea, occurs, or in which
nyctitropie leaves generally abound, it very soon becomcs evident that the peculiarity is
one of degree and not of Hud. The movements under certain circumstances aro
pcrfoinied with exceptional rapidity, and in connection with the incidence of many slight
distm-biug influences; bat in the leaves of Kepiunia tliey may also occur with considerable
readiness and rapidity, and in many other common nyctitropic leaves we find examples
of movements arising under eii-eumstances similar to those under which the movements
of Mùmaa and Sepimin manifest themselves. The movements certainly are noitlier'so
readily evoked nor so rapidly executed as those of Himoea and Nepiutiia, bat tho difleronces
in tlris re.speet wliich are present among individual species of common nyctitropic leaves
ore ahnost as great as those existing between the movements of certain of them and those
of A'iftmk,, and certainly quite as great as those e.xisting between the movements of
A'eptmà and those of Mmos,. Under favom-ablo circiimstanecs, a single light eoneussion
of a distal pinna may bo sufieient to induce complete and very rapid action in a
leaf of il&ioss puiim; under similar eircumstances, slow and Imiited action of a certain
number of pinnules, and possibly a certain amoant of convergent movement in the pinna
subject to concussion, may occur in the leaves of A'cptaaio oUrmm: in PithecohUm,
saman a single concussion fails to produce any appreciable effect, but movements arise
under the influence of a certain iiamber of repeated concussions, tho pinnules
gradually rise, the secondary roehises gradually converge and become depressed, and
the primary petiole converges wliere the primary pulvinus retains its activity; finally in
Cmm aUa very repeated agitation is necessary in order to secm-e the occurrence of any
appreciable movement in the piniije.
Fhenomena demonstrating that particular agencies which induce movements in the
leaves of Mitmsa fmlka produce a like eflect at a different rate in the leaves of other
nyctitropie leaves aro constantly occmring. Separation fi-om the axis under certain ci-cum-
Btances induces rapid and maxhnal movement in the leaves of Mvmorn pucKea and slow
and gradual movements in the ease of other nyctitropie leaves, the displacement, whether
rapid or retarded, tending invariably to the establishment of the nocturnal position. The
impact-of a single heavy di-op of rain is in many eases sufficient to induce maximal
movements in the leaves of Mimom pudim ; a brief exposure to a heavy shower sends the
leaves «i PiUmloUum smnm into the nocturnal position ; and prolonged exposai-e to violent
rainfall causes appreciable movements in the pinnas of Cassia alata. A single light breath
of air may be sufficient to cause maximal movement in M^rmsa pudica; repeated agitation
by a breeze causes the leaves of PHheeohUtnn samm to move towards the noctm-nal position;
and strong wind produces a like effect in the leaves of Cassia ahta. Under exposure to