S i
60
}>air of jñn
t h e racliis.
t h e plane
posterior ai
horizoiitally
TJIE CAUSES OP FLUCTUATIONS IN TÜKGESCENCE
a n o t h e r and vdth the lateral surfaces of
5 somewhat elevated at mi acute angle to
na; are in close contact witli oi
Up to this time the m i d r i b renm
of the rachis, but subsequently depression g r a d u a l l y sets in, so that^ the
i d anterior edges of the lamina come in the first place to lie respectively
superior a n d i n f e r i o r , a n d u l t i m a t e l y to be directed do^vnwards a n d j o r w a r ds
angles of v a r y i n g ( ,f obHquity f r om the line of t h e rachis. (Plate I I , Fig. 2.)
T h e motor organs which determine this complicated series of movements are highly
developed and present certain p e c u l i a r i t i e s which gi-eatly f a c i l i t a t e a comprehension of
t h e i r action. Both superficially and in section it is at once evident that they are
composed of two distinct mas.«es of tissue, one of which is of a w a r m ochreous a n d the
o t h e r of a vivid g r e e n colour, and that these hold certain definite positions in relation to
t h e deeply keeled" vascular bundle of the pulvinus, and to the u n d e r and upper surfaces
o£ t h e base of the outer h . l f of the lamina, -l h e ochreous tissue lies beneath and to
t h e outi-r side of the vascular buncUe a n d spreads out thence over the under surface of
t h e base of t h e outer half of the laumia so as to involve and conceal the origins of
t h e lowest four or tive ribs. The green tissue, on the other hand, lies i n t e r n a l l y and
superior to tlic vascular bundle and for.ns an expansion over the base of the upper
s u r f a c e of the outer half of t h e lamina. The disposition of the two masses of tissue
i s clearly shown in Figure If) of Plate I a n d F i g u r e 3 of Plate II, and the laminar
e x p a n s i o n of the ochreuus tissue i n F i g u r e s 4 and 5 r.f P l a t e I I . _ W e have t h u s to deal
w i t h two di^tinct masses of pulvinar t i s s u e : one intero-sup^
t h e other extero-infeiior,
i n r e l a t i o n to the p u l v i u a r vascular bundle
a n d differing g r e a t l y in th
a n y alterations in the relative
flexible axis must t e n d to give
id t k o base of the outer half of the lamina,
amount of theii- chlorophyll content. Now, it is cleai- t h at
i s t r e n g t h s of masses of tissue so disposed a r o u n d a ceutml,
)vements of divergence or convergence, combined
w i t h v a r y i n g degrees of depression or elevation and of r o t a t i o n i n one or other direction,
a c c o r d i n g as° t h e alteration iinpHes i n c r e a s e d s t r e n g t h in one or the other mass ( P l a t e VIL
F i g . b). Were t h e y merely related to the axial bundle they would give rise only
t o ° d i v e r g e n c e or convergence, depression or elevation, and we should have movements
similar to those which we fiad m a n i f e s t i n g themselves in the secondary petioles of m a ny
b i p i n n a t e leaves; but the movements are ia this case f a r t h e r compHcated b y rotation,
bocause t h e y represent the outcome of forces wliich in the case of bipinnate leaves are
d i s t r i b u t e d between the secondary petioles and the pinnules. The rotation of the axial
b u n d l e is d e t c n n i n e d by the relations which the opposed masses of pulvioar tissue hold
t o the i n f e r i o r and superior surfaces of the rigidly expanded lamina. The oclireous
i u f e r o - e x t c r n a l mass is expanded over the base of the i n f e r i o r sui-facc of the outer half
of t h e lamina, whilst the green intero-superior one holds a corresponding relation to the
u p p e r siirface of t h e same area, and consequently any increase i n s t r e n g t h in the former
mass must t e n d to elevate, and a n y increase in s t r e n g t h in the latter mass must t e n d to
depress, the outer half of the lamina, and therefore to causc corresponding inward or
o u t w a r d rotation or t o r s i o n of the flexible vascular bundle. Were the lamina not rigidly
expanded, we should h a v e h a d movements of elevation a n d convergence of the micbibs and
f o l d i n g u p of the outer haK of the lamina alti-rnaling with movements of divergence and
depression of the midrib and expansion of the lamina ; and as a matter of fact such
movements do a c t u a l l y oocm' at a parliciilar period in the life of the pinna. Wlien.
however, the landna becomes rigidly and permanently expanded, the factors wliich
previously gave rise to movements of f o l d i r g and unfolding do not cease to exist, but
IN THE MOTOR OBGANS OF LEAVES. 61
t h e i r action
correspondi
is now expended in producing general rotation of the lamina and
t(U-.-iion of the flexible vascular axis of the puMnus. Had the pulviuar
pads been disposed iiiferiorly and suppiiorly, we shoidd have had simple elevation and
depression, had t h e y been external and internal simple convergence and d i v e r g e n c e ; but
bccauFe t h e v actually arc extcro-inferior and intoro-supeiior and r e l a t e d respectively to
t h e under and upper surfaces of one-half of the lamina only, we have rotation t a k i ng
place and modifying the character of the movements and the position which they
i i l t i m a t e l y give rise to.
F r om the Jiature of the diurnal and noctumal positions of the pinnaa, it is clear
t h a t d u r i n g t h e day the intcro-superior and during tlie night the extero-infcrior pulvinus
nuist pos,scss a r e l a t i v e excess of s t r e n g t h ; and the conspicuous differences in colour which
t h e y exhibit renders it easy to comprehend why this should be the case. Even
t h e most cursory examination in this case shows t h a t the intero-superior pad is much
r i c h e r in chloropliyll tiian its opponent. This implies a p r o p o r t i o n a t e excess in funct
i o n a l activity, and t h e r e f o r e of turgescence, under the influence of sohir stmndation.
But increased turgescence implies increased strength and resistance in the tissue in
which it occurs, and consequently the intero-superior pad must necessarily acquii-e a
r e l a t i v e increase in power during the day which enables it t e m p o r a r i l y to overcome
The action of its opponent.
J l o v c m c n t s of rotation which are so f r e q u e n t l y present in h i g h degree ia t h e case
of t h e pinnaa and pinnules of n y c t i t r o | ) i c leaves are absent or inconspicuous in the case
of their p r i m a r y and secondary petioles, because in t h em t h e motor apparatus has no
horizontal surfaces like those of the laminan to act upon. Where such surfaces are
))rosent, and the opposed masses of tissue are specially r e l a t e d to them, t h e action of the
f a c t o r s which in the case of the petioles give rise to simple convergence a n d divergence,
elevation and depression is p a r t i a l l y expended in d e t e r m i n i n g various degrees
of rotation. In the present instance rotation advances so far that the outer half of
t h e lamina, and the outer side of the keel of the midrib come to lie directlj'
upwards, which of course implies that the basal laminar expansion of the exteroi
n f e r i o r pulviuar pad lies to the upper side of the base of the midrib. It is now
i n c a p a b l e of giving rise to a n y f u r t h e r r o t a t i o n , and its r e m a i n i n g power is expende
d in producing lateral pressure on the outer, and, for the time being, upper side of
t h e keel of the midrib and proportionate depressive flexion in it.
T h e alternation of the diuinal and nocturnal positions is not caused by altern
a t i n g diurnal decrease and nocturnal increase in the absolute s t r e n g t h of the exteroi
n f c r i o r mass of pulvinar tissue, but by a l t e r n a t i n g diurnal increase and îiocturnal
decrease in that of the i n t e r o - s u p e r i o r one. There is nothing to show that any rise
in turgescence of the mass of tissue making for t h e nocturnal position a t t e n d s removal
of solar stimulation. On the contaary, we iind the same position a p p e a r i n g under the
iiiflucnce of exposure to excessive t r a n s p i r a t o i y loss, and in leaves which are detached
f i om the axis, and t h e r e f o r e subjected to entire arrest of water-supply. The position
is thus clearly one a t t e n d i n g general loss and not any localised rise in turgescence.
On the remoA-al of solar stimulation, a general loss of turgescence, and consequently
of s t r e n g t h , takes placée t h r o u g h o u t the whole pulvinus, but t h e loss is greatest in those
portions which make for the diuinal position, because t h e y are p h y s i o l o g i c a l l y stronger
a n d s t r u c t u r a l l y weaker than their opponents. The result of this is t h a t , although tlio