7 3 THE CAUSKS OP PLÜCTOATIOSS IS T0UGESCENCE
a n d the e a r t h goufly lyasliod away midor a stream of n m n u i g water, so as to f r e e t he
roots ivithout i n j u r y . The entire plant was next weighed, and the weight was a s e e r t a i n cd
t o be only 2-03 grammes. A certain amount of the increment in weight which was
s u s t a i n e d Ijy the acid d u r i n g t h e corn-so of the experiment must, of course, ba erodited
t o the atmospheric h u m i d i t y of the air of the chamber; but as t h e latter was a very
small one, the mass of water f u r n i s h e d by t r a n s p i r a t o r y loss during t w e n t y - f o u r hours
must have weighed nearly twice as much as the e n t i r e p l a n t f u r n i s h i n g it.
Ixperimat 11.—A plant with ÜTO leaves and weighing 2'32 grammes was treated
i n exactly t h e same fashion as that in the previous experiment. At the close of four
hours' exposure t h e sulphuric acid indicated a gain of I'ic.c. of water, equivalent to a
m a s j of more t h a n half the weight of the entire plant. (Vide Appendix B.)
T h e extreme a c t i v i t y of t r a n a p i r a t o r y loss is f u r t h e r very clearly indicated b y the
phenomena which a t t e n d exposure to direct sunlight. When the soil and air are dry
continued exposure to t h e rays of the son is followed by a complete assumplion of the
n o c t u r n a l position b y t h e various parts of the leaves, and, even when soil and air are
boti, oomparutively humid, a v e r y brief exposure to direct insolation is sufficient to give
r i s e to various degrees of elevation of the pinnules. The phenomenon is p a r t i c u l a r ly
s t r i k i n g when it occurs in a large bed of the plant, a portion of which is urtifioially
shaded owing to the contmst presented by tho fully expanded loaves m t l m i the
l i m i t s of the shaded area and those in the insolated one. Taldng the expelimenta
a n d natural data together, they appear very cleai'ly to incUcate that the dim-iiai
p o s i t i o n of the leaves is one of v e r y unstable e i i d h b r i m n dependent on a defimto relation
between supply and loss oí wnter, and liable t o be d i s t u r b e d by any eonditioM i m p l p ng
e i t h e r diniimition i n supply or increase in loss. The phenomenon of p a r t i a l assumption
of t h e nootmnal position by tho pinnules under the influence of direct insolation, even
where the h u m i d i t y of the soil is v e r y high, may be taken as i n d i c a t i n g the existence
of a d e i n i t e limit t o the a c t i v i t y of root a b s o r p t i o n ; bat f r om expenmental data, wlnoh
will be f a m i s h e d p r e s e n t l y , it appears not unlikely that another factor comes into p l ay
t o produce the result. The elevation of tcmporatm-e which attends dn-ect insolation not
o n l y gives rise to increased ti-anspiratory loss, but must also cause an expansion of tho
.raseous contents of t h e water-conducting system which m a y well act as an obstruction
t o the passage of Hquid along i t ; so t h a t the d e p a r t u r e f r om t h e diurnal position may be
owing, not merely to increased transpii-atory loss, but to diminished supply of water.
T h a t ° ' t h i s is actually the case is certainly suggested b y the fact t h a t direct insolation,
even within a closed moist chamber, is a t t e n d e d b y a certain amoant of elevation of tho
pinnules Hero, of course, the atmosphere is excessively humid, and t r a n s p i r a t o r y loss is
correspondingly r e d u c e d ; but assimilatory • dccomposilion of water continues, and if tlus
coincides with obstruction to tho passage of liquid along the water-vascular system, a
c e r t a i n amount of decrease in tm-gescence must necessarily ensiie. Tho decrease, however
is inconsiderable, and under normal circumstances, neither alone nor m co-operation
w i t h decrease duo to coineideiitly increased t r a n s p i r a t o r y loss, does it suffice t o determine
t h e full assmnption of the noctm-nal position. It is only under t h e eomc.dent influences
of di-y soil, dry air, and insolation that tho fall nocturnal position is spontaneously
a t t a i n e d d u i i n g the d a y : in other words, it is only attained under cii-oumstancos which
i m p l y dofeotive root-supply, gaseous obstruction of the wator-vascular system, and
excessive transpii-atory loss.
IN THE MOTOR OBGANS OF LEAVES. '
T h e position of the various parts of v e r y y o u n g leaves, in which periodic movements
h a v e not yet bcgnn to manifest themselves, is as f o l l o w s : — T h e primary petiole is
e x t r e m e l y divergent from the line of the axis; so that, whore tho latter is vertical,
i t is either slightly ascending, horizontal, or s l i g h t l y depressed (Plate I, Figs. 11,
12 IS • P l a t e V, F i g . 14). The pinnaj are closely convergent and bent backwards
a t an acute angle over tho upper surface of the p r i m a r y petiole, the rachises of the
basal pair- usually having their upper sm-faces directed more or less inwards, so
t h a t the edges of the closely-folded pinnules are directed ohKqucly outwards and
i n w a r d s in place of directly upwards and downwards, and the distal pinnai are
overlapped and partially concealed. The pinnules, which from tho outset are f u l ly
expanded, are closely convergent, and have their midribs dii-ected upwards and
f o r w a r d s at an acute angle to t h e line of the secondary rachisos. As t i m e goes on, the
p r i m a r y petiole lengthens, but retains its previous position to the axis, and the pinme
g r a d u a l l y become i r s t erected and t h e n bent backwards, so as to lie in the same plane as
t h e p r i m a r y petiole or s l i g h t l y ascending from it, and at tho same t i m e become slightly
divergent (Plato I, Figs. 11, 13, 18). When periodie movements first appear t h e y arc for
a tim°o confined solely t o the p r i m a r y petiole, which d m i n g t h e d a y undergoes elevallon or
ronver=-ence t o w a r d s tho axis, and n o c t u m a l l y r e t u r n s to its original position of extreme
diver=-cnco or sh..ht deflection. Tho amount of dim-nal displacement is at first v e r y small,
but undergoes rapid increase; and, with this, movements determining diurnal divergence
a n d n o c t u r n a l convergence of t h e pinnas begin t o manifest themselves. Finally, movements
appear in the pinnules also, leading to steadily increasing diurnal divergence and
depression and t o a c e r t a i n amount of r o t a t i o n downwards of the posterior or outer half of
t h e lamina a l t e r n a t i n g with noctm-nal convergence, elevation, and reverse r e t a t i om When
movements are at a maximum, the dim-nal and noctui-nal positions of the various p a r ts
of tho leaves are normaUy as f o l l o w : - D i m - n a l l y the primary petiole usually lies at an
ucutc anglo to the axis, but t h e degree of eonvergenco varies g r e a t l y in d i f l e r e n t instances
a n d under differing conditions, owing to the influence of variations in the amount of
t h e leverage exerted b y the distal p a r t s of the leaf. Tho pinme he ahnost in the some
plane as the p r i m a r y petiole or s l i g h t l y ascending f r om i t , a n d are widely d i v e r g e n t ; the
divergence of the basal ones b e i n g f r e q u e n t l y so great as to exceed a right angle, so t h at
t h e y are dii-ected s l i g h t l y backwards ( P l a t e V, F i g . 12). The pinnules lie with their upper
surfaces f a c i n g d i r e c t l y u p w a r d s and their midribs widely divergent (Plate V, Figs. 2, 4).
T h e divergence varies in d i f f e r e n t parts of t h e pinna, the lower pinnules being f r e q u e n t ly
d i r e c t e d somewhat backwards, the central ones at right angles t o t h o secondary raclns, and
t h e distal ones inclining f o r w a r d s . As tho pinnules are set on v e r y closely, tho i n t e r v a ls
between t h em falling v e r y considerably short of tho b r e a d t h of individual laminie, it is
v e r y seldom that tho e n t i r e surface of a n y pinnule is e n t i r e l y exposed, and, as a rule, a
c e r t a i n amouat of the anterior half of each successive l a m i n a is concealed by tho posterior
half of the succeeding one (Plate V, F i g . 3). In the maximal n o m a l noctmmal
p o s i t i o n determined solely by absence of photic stimulation, tho primary petioles
lie nearly at right angles to the axis; that is, in eases where the latter is vertical,
some are horizontal, others sKghtly asoending and still others slightly descending.^ Tho
secondary raeliises are closely convergent and slightly elevated, and the lower pair- are
r o t a t e d , so t h a t their- upper surfaces ore directed more or less inwards. The pinnules are
closely convergent, so t h a t the two rows are in contact w i t h one another, the upper surfaces
of the lamina; f a c i n g d i r e c t l y inwards and the lower ones dii-ectly outwards (Mate V,