674 TRANSACTIONS OF SECTION K. 
destroys its vitality. This is especially true of such seeds as those exposed 
to a drier atmosphere. 
7. The Perception of Light in Plants. By Haroutp Wacerr, I’.2.S. 
It is well known that various plant organs exhibit a definite response 
to the action of light. Free-swimming organs, such as zoospores, move 
towards or away from the source of light. Orthotropic organs, such as 
young seedlings, stems, and roots, bend towards or away from the light, 
whilst diatropic organs, such as foliage leaves, place themselves at right 
angles to the direction of the light. 
It was shown by C. and F. Darwin in 1880 that the illumination of one 
. part of the sensitive organ determines the movement in another part; that 
one part of the sensitive organ perceives the light and that a stimulus is 
set up which is transmitted to another part, where the movement takes 
place. In ordinary foliage leaves the leaf blade is usually the perceptive 
region, the movement being brought about by the petiole. 
It is clear that the object of these heliotropic movements is either to 
protect the plant from a too intense light or to bring it into such a position 
that it can take the fullest advantage of the light which falls upon it. 
There can be no doubt that this is effected with considerable precision. 
But how it is that the plant is enabled to perceive that it is or is not in 
the right position, and the means by which the light stimulus is set up, 
are not yet clearly understood. 
In the case of free-swimming organisms, such as Chlamydomonas, 
Euglena, &c., it is probable that the eyespot or its equivalent is the per- 
cipient organ. The light rays absorbed by the eyespot are those which 
are functional in heliotropism, and these may act in some way upon the 
flagellum either directly or through the cytoplasm, and so bring about the 
necessary modification of its movements by which the course of the organism 
is changed. 
In dia-heliotropic foliage leaves Haberlandt suggests that the epidermal 
cells or special modifications of them function as ocelli or rudimentary eyes 
by converging the light so as to bring about a differential illumination of 
the cytoplasm on the basal wall of the epidermal cells, by means of which 
the stimulus is set up. 
The layer of cytoplasm lining the epidermal cells would, therefore, in 
this case be the percipient organ. The evidence in favour of this view is, 
however, not very satisfactory and has been recently criticised, both on 
morphological and physiological grounds. 
As an alternative to this view, it is suggested that the chlorophyll grains 
are the percipient organs. It is well known that they are sensitive to 
light, and that in some cases they show this by definite responsive move- 
ments. The unequal illumination of the chlorophyll grains when the rays 
of light fall upon the leaf in a slanting direction would be sufficient to 
account for the generation of the stimulus, and it is possible that the 
optical behaviour of the epidermal cells may be of importance in this 
respect. The rays of light which are absorbed by the chlorophyll are the 
only ones which appear to be functional in heliotropism, and these by 
their action upon the various colouring matters contained in the chlorophyll 
may set up in the cytoplasm immediately adjacent changes necessary 
to bring about the stimulus. 
MONDAY, AUGUST 30. 
Joint Discussion with Section B and Sub-section K (Agriculture) on 
