96 
REPORTS OF SOCIETIES. 
Apr., 1890 . 
structure of the green parts ; secondly, in the position of these parts. 
The leaves of plants have ordinarily one surface facing upwards to the 
sky, the other downwards to the earth. In this position (diahelio- 
tropic) the upper side receives far the greater amount of light, and, to 
guard against waste of chlorophyll, a special structure (palisade 
tissue), consisting of elongated upright cells, is found on the upper 
surface. In these cells the cliloroplastids, being discoidal, and not 
spherical, in form, imbedded, as they are, in protoplasm irritable 
under strong light, can be so turned as to expose only their edges to 
direct rays, which they could not be in the ordinary shaped cells of 
the lower side of the leaf. In shade-growing plants, such as ferns, 
these special cells are entirely absent, both surfaces of the leaf being 
composed of similarly formed cells. Again, by the motion of growth, 
plants have the power of placing their leaves in the position best 
adapted to receive such amount of light, neither more nor less, as 
will ensure the best economy of their chlorophyll. For instance, 
observe plants growing in a crowded hedge—stems are curved, posi¬ 
tions of leaves altered so as to get the best light for that particular 
plant—a manifestation of heliotropic irritability. In plants growing 
in exposed situations, under an extremely powerful sun, the exposure 
of the full surface to the direct rays would occasion a far too wasteful 
expenditure of the precious chlorophyll. Hence in these a different 
position (which may be called paraheliotropic) is often found, in 
which the edges of the leaves, and not the two surfaces, face directly 
up and down. A good instance of this is shown by the Eucalyptus 
(Australian Blue Gum), in which the leaves of the lower branches, 
growing under the shade of the surrounding forest, and therefore in a 
modified light, are horizontal (dialieliotropic), while the upper 
branches, towering aloft in the full blaze of the sunlight, are all 
turned edgeways to the zenith (paraheliotropic), and so only catch the 
slanting rays at morning and evening. This explains the strange 
phenomenon of the so-called compass plants, whose habit of growth 
is such that they so twist themselves to expose only the edges of their 
leaves to the direct light, that these edges always he north and south 
in the meridian of the place where they grow, while the exposure of 
the sides is due east and west, by the knowledge of which fact the 
cunning botanist may steer an accurate course. These, when the 
growth of the leaf ceases, have attained their “ fixed light position,” 
as it is called. But there are other plants which can alter the posi¬ 
tion of their fully developed leaves at any time, according to the 
varying conditions of the light—bright, dull, day, or night. (Growing 
specimens of several of these were shown, Oxalis, Acacia, Bauhinia, 
folding, drooping, or retracting against each other the edges of the 
leaves, according to the strength of the light to which they were ex¬ 
posed ; and so presenting, according to its conditions, more or less of 
surface to its direct action.) This has been called, by a poetic figure, the 
“diurnal sleep” of plants. Many such, fora different reason, droop, 
contract, &c., at night, in this case to protect themselves, not against 
undue intensity of light, but against perils of night frost and undue 
radiation of stored up heat. The general conclusion resulted that 
each particular plant has become attuned to the particular conditions 
of intensity of light suited to its natural development, and that neg¬ 
lect of these conditions adds greatly to the difficulty of securing the 
healthy progress of any plant in new environments. At the conclu¬ 
sion of the meeting, the Society, through the President, offered its 
hearty congratulations to Mr. M. S. Pembrey, one of its members, on 
his recent election to a Radcliffe Travelling Fellowship. 
