76 
JOURNAL OF THE EOYAL HOETICULTURAL SOCIETY. 
and erect, as in CampaiiuJa rotundifolia, the common Harebell. 
Experiments have shown that if the plant be grown in semi -obscurity, 
the stem leaves will grow like the first formed, w^ith broad blades and not 
linear— probably in consequence of growing less vertically. 
Besides the obvious fact that these differences of form are correlated 
to the habits of growth respectively, Mr. Scott Elliot has proved experi- 
.mentally that as a rule length increases in shade and moisture, so that 
the ratio of the length to the breadth is always less in exposed leaves 
than in shaded ones.* This appears to apply to naturally broad leaves. 
Similarly in monocotyledonous aquatic plants the submerged leaves 
growing erect, being supported by the water, are long and narrow, but 
as soon as they reach the surface, and either float upon it or are erected 
into the air, they acquire broad blades, as Potamogeton heterojjhTjlUirn, 
Sagittaria sagittifolia, &c. 
Light cannot be separated from heat in the rays of the sun which fall 
upon plants, and we find that a linear leaf-blade may be imitated by 
another structure also placed in a vertical position as a protection against 
injurious influences. Thus by suppressing the horizontal leaflets and 
developing the leaf-stalk into a phyllode, as in Australian Acacias, the end 
is gained of guarding against injury by radiation. In Gum trees this is 
done by clothing the tree with scimitar-shaped leaves, which hang 
vertically. In some cases the leaves are all suppressed, while the stem 
itself acquires green wing-like appendages, which answer all the purposes 
of leaves, but are, of course, vertical. 
Effects of Light on Flowees. — A very obvious and often noticed 
difterence exists between the bright colours of flowers of Alpine and 
Arctic regions and those of lower latitudes and altitudes. This is due to 
the extreme brilliancy and continuance of the light and clearness of the 
atmosphere in those regions, as has been proved by experiment. For 
when plants were transferred to them from, say, the neighbourhood of 
Paris, their flowers acquired a similar brightness when they were grown 
with the native floras. 
Conversely in this country the colours of many flowers, especially 
crimsons, were much altered to paler hues in the too intense heat of the 
late dry seasons, showing that there is probably an optimum for every 
kind of flower. 
The fact that the common pink Lilac, Avhen forced, is white-flowered, 
while a purple Hyacinth allowed to blossom in total darkness may be still 
purple, results from the fact that while the latter is well provided with 
properly organised food, upon which the colouring matter of the flowering 
depends, the Lilac has not been provided with it, or at least suffi- 
ciently so. 
Buds on Boots. — Boots, as a rule, do not produce leaf -buds, but 
when they become exposed to light they are much more inclined to 
develop them, as may be seen in any hedgerow where Elm roots are 
exposed. The ordinary behaviour of roots is to grow towards the darker 
side and avoid the light, as in Ivy growing against a wall. A curious 
instance of want of light is the production of tubers. Thus a Potato, if it 
* "The Effect of Exposure on the Eelative Length and Breadth of Leaves," 
■Journ. Linn. Soc, xxviii. 18D1, p. 375 ; see Origin of Plant Structures, p. 239. 
