THE CAUSES OP PLUCTUATIONS IN TUEGESCEXCE
fluctuations in tho r a t e of t h e i r m a n u f a c t u r e are at once followed by conspicuoua altera
t i o n s in t i n t and t e x t u r e . In a certain n u m b e r of cases, however, t h e y ai-e stable, and
h e r e an entire cessation of protoplasmic a c t i v i t y may occur without giving rise to any
a p p r e c i a b l e results. In t h e fii-st instance, d e a t h gives riae to i n i m e d i a t e a l t e r a t i o n s i n colour
a u d t u r g e s c e n c e ; in the l a t t e r , it produces n o appreciable efFcct. Turgescence is n o r m a l ly
s o l e l y d e p e n d e n t on c e r t a i n properties of t h e cell-sap; aud, so l o n g as these arc r e t a i n e d , it
is a m a t t e r of i n d i f f e r e n c e w h e t h e r l i n n g protoplasui be present or not, or what changes
i n f o rm it may u n d e r g o . In so f a r as i n t r i n s i c conditions a r e conoemod, it is loss of
osmotic p r o p e r t i e s in the cell-sap, and not a n y dh-ect a l t e r a t i o n in t h e s t r u c t u r e of the
protoplasm, t h a t gives rise to loss of t u r g e s c e n c e : but, in dealing with a n y experiments
on tissues in which the m a i n t e n a n c e of turgescence is n o r m a l l y related to continued
v i t a l i t y , we h a v e c a r e f u l l y to bear in mind tliat c e r t a i n extrinsic conditions may come
i n to m o d i f y t h e n a t u r e of the results a c c o m p a n y i n g a cessation of f u n c t i o n a l a c t i v i t y.
I n cases w h e r e t h e cessation of f u n c t i o n a l m a n u f a c t m ' o of osmotic products is accomp
a n i e d by t h e i n t r o d i i c t i o n of extrinsic products (as i n experiments wliere the leaves of
Kalanchoe or the flowers of Hibiscus are exposed to an a t m o s p h e r e of ammonia), t h e loss
of turgescence a t t e n d i n g t h e d e a t h of t h e tissues is reduced to a m i n i m um ; whilst, in
cases where no such m o d i f y i n g influence is present, t h e amount of loss will bo d e t e r m i n ed
s o l e l y by tho extent to w h i c h a n y stable osmotic m a t e r i a l s of i n t r i n s i c origin are present.
I n all such e x p e r i m e n t s we h a v e to consider the influence of tho conditions to w h i c h the
t i s s u e s a r e exposed in p r o d u c i n g t h e f o l l o w i n g e f f e c t s :—
I s i — C e s s a t i o n of f u n c t i o n a l a c t i v i t y.
2nd—Addition of extraneous osmotic c o n s t i t u e n t s to tho cell-sap.
SrJ—Distm-bances in t h e r e l a t i o n s of s u p p l y and evaporative loss of fliiid.
4 f / i — A l t e r a t i o n s i n t h e filtrative power of the protoplasm, and possibly of the
cell-walls.
I n cases where leaves such as those of Kalanchoe, or flowers such as those of
ITibiscus rosa-sinensis, are exposed to an atmosphere of ammonia ia a moist chamber, and
w i t h the bases of their stalks s u b a q u e o u s l y d i v i d e d and immer.sed in water, we find turgescence
r e m a i n i n g almost \ i n a f f e c t e d b y the d e a t h of the tissues, because of the a d d i t i o n of
e x t r a n e o u s osmotic materials to the cell-sap co-existing with f r e e s u p p l y and abolished
e v a p o r a t i v e loss of fluid. "Where simOar tissues are exposed t o an atmosphere of chlorof
o rm u n d e r otherwise like conditions, an abolition of turgescence follows t h o cessation of
f u n c t i o n a l acti-vity, because the f r e e s t s u p p l y and abolished e v a p o r a t i v e loss of water will
not make up for the loss of osmotic properties in tho cell-sap in resisting the elastic
r e c o i l of the cell-walls. Where, as in experiment X X I of chapter I I , the same tissues aro
exposed to an a t m o s p h e r e of ammonia, without special provisions for s e c u r i n g s u p p l y and
a b o l i s h i n g e v a p o r a t i v e loss of water, a v e r y considerable loss of turgescence takes place in
s p i t e of the a d d i t i o n of extraneous osmotic m a t e r i a l s to tho coil-sap ; and finally, where
t h e y are exposed to the action of osmic vapour, the loss i n turgescence associated w i th
a b o l i t i o n of f u n c t i o n a l a c t i v i t y is r e t a r d e d , a p p a r e n t l y owing t o the i n t r o d u c t i o n of obstacles
t o filtration. Such obstacles do not, however, arrest tho loss of t u r g e s c e n c e ; t h e y merely
r e t a r d it, whereas t h e i n t r o d u c t i o n of extrinsic osmotic m a t e r i a l s b r i n g it to a standstill
w h e n e v e r t h e i r capacities come to e q u i l i b r a t e those of tho elastic recoil of t h e cell-walls.
T h e effect of loss of osmotic p r o p e r t y in the cell-sap may be r e t a r d e d b y obstacles to
IN THE MOTOE ORGANS OF LEAVES. 37
filtration ; but wc have no positive evidence to sliow t h a t any loss of turgescence ever
arises owing to the development of special filtrative facilities in the protoplasm apart
f r o m alterations in osmotic p r o p e r t y of tho cell-sap. Tho elastic recoil of the cell-wails
i s the essential factor in m a i n t a i n i n g tho pressure which makes for filtration and is
c o n s t a n t l y s t r u g g l i n g w i t h the osmotic capacities of the cell-sap. "When a condition of
e q u i l i b r i um h»is been established b e t w e e n these opposed agencies, it will remain stable so
l o n g as t h e y remain unaltered, but will be d i s t u r b e d by a n y t h i n g a f f e c t i n g t h e i r mutual
r e l a t i o n s . Any rise i n osmotic capacity of tho sap will thus at once tend to increase
turgescence, and any rise in external pressure will tend to diminish it ; and it is
o n l y where fluctuations of equal value occur in both simultaneously that turgescence
will r e m a i n unaltered.
CHAPTER IV.
^he ¿xtvinsic a^mts affecting iiiugesccncf.
I n the previous chapter we have seen r e a s o n to believe t h a t turgescence is essentially
d e p e n d e n t on certain osmotic properties in t h e cell-sap which cause the cell cavities to
i n c l u d e more fluid than their passive structm-al peculiaiities will account for. We have
n o w to consider t h e influence of c e r t a i n e x t r i n s i c f a c t o r s i n a f f e c t i n g t h e actual degree of
t u r g e s c e n c e present at a n y p a r t i c u l a r time. Tho f u n c t i o n a l activities of the p r o t o p l a s ts
of the tissues provide that tho cell-sap shall have certain osmotic capacities, but the
d e g r e e of turgescence at a n y t i m e must be affected b y the extent to which the l a t t e r are
able to s a t i s f y themselves. Protoplasmic a c t i v i t y secures t h e presence of osmotic products,
but external conditions must determine tho extent t o which these will be able to give
r i s e to turgescence. The n a t u r e and q u a n t i t y of osmotic materials in a mass of tissue
m a y reniain unaltered ; a n d yet, in spite of this, its degree of turgescence may v a ry
g r e a t l y , owing to v a r i a t i o n s in the amount of water available for them to act upon.
T h e total mass of water contained within any t e r r e s t r i a l p l a n t is subject to constant
v a r i a t i o n s which are due to fluctuations in r o o t - s u p p l y a n d evaporative loss ; and where
t h e former of these is d e f e c t i v e or the l a t t e r excessive, the highest degree of protoplasmic
stimulation and activity will be unable to m a i n t a i n the turgescence of tho
tissues to its normal extent. Solar s t i m u l a t i o n c e r t a i n l y t e n d s to establish the maximum
of turgescence in c h l o r o p h y l l - c o n t a i n i n g tissues where sufficient water is available to
a l l ow t h e m to s a t i s f y t h e i r osmotic p r o p e r t i e s ; but the s t r o n g e s t solar stimulation will
not suffice to prevent loss of t u r g e s c e n c e under conditions of defective supply or
excessive loss of water. E v e r y one, and specially every one who has lived in the
tropics, must be familiar with t h e phenomena of t e m p o r a r y w i l t i n g i n c e r t a i n common
leaves, and the assumption of the n o c t u m a l position by n y c t i t r o p i c ones, which so
c o n s t a n t l y manifests itself in the early p a r t of the a f t e r n o o n d u r h i g hot d i y weather.
T h e two phenomena have been by some observers r e g a r d e d as the results of two
d i s t i n c t causes, willing of common leaves being allowed to be t h e result of deficient
w a t e r - s u p p l y , * whilst tho movement in n y c t i t r o p i c leaves is ascribed to the direct
a c t i o n of excessive solar s t i m u l a t i o n , f but tho following experimental data clearly
• Sachs, Vorlosuug, XIV. s. 274.
t Snchs, Vorlesuag, XÏXVI. s. 767.