, 1913-08-20

1880 that liz grass seedlings 'sensitlveness
to light is sometimes confined to II small
part Of the plant; and that this part, when
stimulated by light, transmits an intuence
to distant parts, exciting them to bend
1t is now also we'll known that the stimu-
luS which causes the foliage of leaves of
other pants to place themselves by means of
movements of their leuf-stalks i'll such II
position rs to receive the fullest advantage
from the incident rays of hight, is perceived
largely by the leaf-blade, or, in some cases.
by the upper part of the leaf stalk, and that
this stimulus is transmitted to another
part Of the leaf stalk, by which the tuning
Of the leaf is brought about. II might be
asked whether the plant does actually poss-
ess any means corresponding to the ner-
vous system of animals by which such u
stimulus could be transmitted, aud the an
swer to this would be that, so far as we
know at present, there is no definite nerve
organisation or tissue ia plants, but that
there is no reason why the stimulus should
not be transmitted by the living substance
(protopIasm). Although the cells of plants
are apparently completely separate from
one another by their non-living cell wallis,
yet they are oil connected together by Il sys-
tenn Of excessively delicate ibrlls of proto-
plasm, which pass from one cell to the next;
through very minute pores in the cell wall.
All parts OF the living substance of u plant
therefore, are placed IN communication by
means of these fine threads, and there is no
reason why this should not form a perfectly
efficient apparatus for the transmission Of
stimuli, That light stimuli and other stimuli
are transmitted there seems to be no rea-
souable doubt. Ii, for example, the SUI face
of Il suitable leaf is exposed to light striking
1t obliquel~, whilst the leaf-stelk IS kept ub.
SoluteIy IN the dark, it Will be found hat
the l~oaf-sfalk bends IN such a way as to-
carry the leaf blade into the optimum light
position. Here, then, the leaf-hlade acts
as the precIplent organ, the light stimulus
being then conducted from the ieaf-blade To
THAT part of the Ieat-staik where the bond
ing takes place. In some leaves there is u
special provision nude for THIS, in the form
OF u swollen sensitive portion called the pull
Unns, at the base of the leuf-stalk Just
where it Joins the stem. The stimulus
which is set up in the leaf blade is probably
the result of chemical changes in the chic
rophyll caused by the light. Careful exam
nation shows that the chlorophyIl corpus
cIes absorb more especially the red, blue.
and violet rays of the mixed white light that
falls upon them. These rays do undoubted-
~ set up chemical changes us Il result of
their absorption by the chlorophyll, and 1T
Is only the rays thus absorbed, especially
the hlne and violet rays, which are capable
OF causing the leaf to turn. Thus it is found
that when plants are illuminated by blue or
violet hight the leaves turn readily, but when
they are Illuminated by other rnys, there ~
little or no turning SO, again, if we place
as u screen a green solution of chloropblll
between the plant and the light, so rs to eut-
off the rays that nre absorbed by chloro-
phyIl, then no turning takes place, although
the other rays pass through quite freely to
illuminate the plant.
A further piece of evidence in favour of
the view that it is the light absorbed by the
chlorophyll grains that sets up the stimulus
is found in the fact that Ia many plants, the
chlorophyll corpnscles are themselves; under
the Influence of too strong or too weak Il
light, brought into more suitable positions
in relation to the tight either by being turn
ed completely round So as to present Il great
er or less surface TO it, or by being moved
in to different positions in the cell. AS soon
as the chlorophvll grains are in the right
position the movement stops, and tho stun
ulus to movement must also have ceased.
1t is the same in the turning of the leaf.
SO long as the leaf is in IN obique position