I '1
ÔS THE CAUSES OF FLUCTUATIONS IN TUEGESCEXCE
v a i y with the Ftnictural f a c i l i t i e s provided for filtration. The pulviiiar tissues i n Lcucoena
qlauca, and especially in Mimosa pudica, provide great f a c i l i t i e s f o r filtration; their pulvinar
tiirgosconcc is conscqueutly very imstable, and moveuieuts are very r e a d i l y induced in
t h e m ; ^vlnlst in Cusiia alaia tlie tissues p r o v i d e minor facilities, a n d must be s u b j e c t ed
t o f r e q u e n t l y r c c i i r r i n g increments of external pressure before a sufficient filtrative
displacement has occurred to cause a l t e r a t i o n s i n turgescence of sufficient m a g n i t u d e to
lead to appreciable alterations in the relative positions of the various parts of the
l e a v e s .*
P a r a l l e l phenomena present themselves in cases where the tissues, in place of being
exposed to conditions t e n d i n g to give rise to increased filtration, have t h e i r normal waters
u p p l y interfered with. If a shoot of Lcucoena glouca or MimoS'i pudica be gently
d o t a c h e d fr.im the axis with a careful avoidance of all agitation likely to induce
i n c r e a s e d pulvinar p r e s s u r e a n d consequent aiigmented filtrative loss, the jjrecaution does
not under nonnal circumstances prevent t h e leaves f r o m m a n i f e s t i n g movements, very
r a p i d l y in t h e f o r m e r and almost i m m e d i a t e l y in the l a t t e r case ; whereas in Cassia alaia
movemonts make theii- appearance only v e r y gi-adually, and a f t e r the lapse of a considera
b l e interval. In all cases t h e deprivation of w a t e r - s u p p l y under such circumstanccs is
alike, b u t t h e r a t e of movement subsequent to its incidence v a r i e s a c c o r d i n g to t h e a c t i v i ty
w i t h which filtrative processes ai-e n o r m a l l y carried on by the p u l v i n a r tissues, The
filtration which is c o n s t a n t l y going on in the p n h ^ n a r tissues of Lcucoena glauca and
Mimosa pudica under n o n n a l circumstances is more r a p i d t h a n in those of Cassia alaia;
a n d consequently when the water-supply is cut off t h e t u r g e s c e n c e of the tissues in
t h e former cases f a l l s m o r e i-apidly t h a n it does in t h e l a t t e r one.
T h e degree of turgescence in any mass of tissue at a given time must neccssarily
d e p e n d on its osmotic c a p a c i t y a n d on the supply and filtrative loss of fluid to which
i t is subjected. The condition is one of u n s t a b l e equilibrium, and the degree of instab
i l i t y must necessarily be r e l a t e d to the degree to which i n h e r e n t filtrative facilities are
p r e s e n t . Where these are present in high degree, any d i m i n u t i o n in osmotic power,
a n y increase in conditions of external pressui-e, or any arrest in the supply of fluid
a v a i l a b l e for absorption, will give rise to p r o p o r t i o n a t e l y rapid loss in turgescence.
AVhere, on the other hand, t h e y are low, the loss must necessarily be con-espondingly
d e l a y e d . Turgescence implies a balance between certain conditions of osmotic g a i n and
filtrative loss of fluid, and any decrease in the former or increase in the latter must
n e c e s s a r i l y t e n d to give rise to diminished turgescence. Decreased absorption may arise
e i t h e r as the result of decreased osmotic p r o p e r t y or of decrease in t h e amoimt of fluid
a v a i l a b l e f o r absorption, a n d increased filtration must t e n d to arise under the influence of
a n y a u g m e n t a t i o n of external pressure. 'l"he structural facilities for filtration, however,
r e m a i n constant, and consequently the a l t e r a t i o n s in t u r g e s c e n c e which a c t u a l l y t a k e place
u n d e r the influence of any alterations in the conditions to which a mass of tissue is
exposed must n e c e s s a r i l y differ in d i f f e r e n t instances, their m a g n i t u d e and the rate at
w h i c h t h e y are capable of i n d u c i n g appreciable movements v a r y i n g with the s t r u c t u r al
p e c u l i a r i t i e s of t h e a f f e c t e d tissue.
• According to Soeis (Vorlesangen XV, s. 302) more agifntion Tiot improbably raises transpirafory loss from the lenyej ;
lint sucli incrcaseH loss cannot be the essential dctermiDant of thf alterations in tiireeseence of th® irotor organs alladed to
abore, as we find the assùmption of the nocturnal position ocoarring equally readily wliere the wind is accompanied by heavy
rainfall, or as tbe result of agitation by tlie latter alo.,e.
I!i THE MOTOR ORGiîîS OF LEAVES,
CHAPTER VITI.
'eKfi nijctitropii; inobiDTcnts of partiaUdr Unbts in rclniton to i\u
structxrve anii avrangcincnt oi lltc iissitcs in titeiv ^lotor ovgvins.
"Ulu luoliemcnis of \ht g^bcs of Ciissia nlata anil
(Eassia ^umatcana.
I n the course of the p r e c e d i n g chapter, an attempt has b e e n made to show that the
occurrence of n y c t i t r o p i c movements g e n e r a l l y is d e p e n d e n t on the presence of opposing
masses of tissue situated in the motor organs and differing f r om one another in
f i m i t i o n a l and sti-uctural s t r e n g t l i . If this h e a c t u a l l y t h e case, it ought to be possible
t o show that some definite r e l a t i o n exists between the special movements exhibited by
t h e leaves of p a r t i c u l a r p l a n t s and c e r t a i n peculiarities of structm-al detail and arrangement
in the tissues of their motor organs. In a t t e m p t i n g to do this, the best CDurse
appears to be t o t a k e a c e r t a i n number of i n d i v i d u a l leaves a n d consider the peculiarities
of their movements and t h e s t r u c t u r e of t h e i r motor organs i n detail.
T l i e leaves of Cama alata afford specially favourable o p p o r t u n i t i e s for s t u d y , because
of their large size and the h i g h l y developed and complicated movements which they
exhibit. F\dly developed leaves in this species have f r om nine to twelve pairs of
p r a c t i c a l l y sessile pinnai, t h e lowest pair b e i n g of r e l a t i v e l y small size a n d inserted close to
t h e base of the leaf and at a considerable distance f r o m the n e x t p a i r . During t h e period
i n the life of a leaf in wliich movements are at a maximum, t h a t is, i n f u l l y developed
but still comparatively young leaves, the diurnal and nocturnal positions of the pinnas
a r e as follows : — D i u r n a l l y t h e i r superior and inferior surfaces look d i r e c t l y upwards and
downwards ai^d lie a p p r o x i m a t e l y in t h e same plane with the upper face of the rachis,
t h e i r midribs at the same time d i v e r g i n g almost at right angles to it. (Plate I I , F i g . 1.)
I n the early part of t h e n i g h t , on the other hand, the upper and under surfaces of the
laminai look d i r e c t l y inwards and outwards in a plane at right angles to t h e upper face
of the rachis, and the midribs are closely convergent and directed downwards at an
a c u t e angle to it. (Plate II, Fig. 2.) It is clear t h a t t h e movements which lead to tlie
a l t e r a t i o n of these positions must consist of a l t e r n a t i n g convergence and divergence of the
midribs of the pinnaj, combined with excessive r o t a t i o n in opposite directions and a
c e r t a i n amount of elevation and depression.
If we watch the actual progress of the transition f r o m t h e diurnal to the n o c t u r a al
position, we find that the first indication of displacement lies in a c e r t a i n degree
of elevation of the lamina, so t h a t its upper surface slopes u p w a r d s f r o m t h e plane of the
rachis. At t h e same time the divergence of the midribs g r a d u a l l y decreases. The general
e l e v a t i o n of the lamina is succeedcd by steadily i n c r e a s i n g elevation of its posterior or
outer half over the level of the anterior or axillary one, which necessarily implies
corresponding r o t a t i o n of the m i d r i b . Rotation and convergence n o w advance simultaneo
u s l y until the u]ipcr sm-face of the lamina has moved through a quadi-ant and the
m i d r i b has converged so g r e a t l y t h a t the upper surface of the opposing members in each
ANN. HOY. B o r , GARD. CALCUTTA Voi.. V I.