THE LADIES' MAGAZINE OF GARDENING. 313 
in a very striking manner. Over the vast forests of that country, clouds 
sometimes spread and continue for many days, so as almost entirely to 
intercept the light of the sun. In one instance, just about the period of 
vernation, the sun had not shone for twenty days, during which time the 
leaves of the trees had reached nearly their full size, but were of a pale or 
whitish colour. One forenoon the sun broke forth in full brightness ; and 
the colour of the leaves changed so fast, that by the middle of the afternoon 
the whole forest for many miles in length exhibited its usual summer 
dress. Of this local action of light the gardener avails himself on many 
occasions ; and by various modes of excluding this agent from particular 
parts of plants, so as to effect their etiolation, he is enabled so far to modify 
or change, not only their colour, but their more active properties, as in 
some instances to improve their natural qualities as articles of food, and in 
others to deprive them of those which might render them unsavoury or 
unwholesome. 
The progress of colouration in an etiolated leaf, when exposed to sun¬ 
shine, was observed by Senebier. The most tender parts first pass from 
white to yellow, the yellow then becomes deeper; next, some green spots 
appear on different parts, which multiply, extend, and meet, till the whole 
exhibits a green colour. This progressive colouration is effected exteriorly 
by the action of light, and is independent of the internal vegetation of the 
organ. (Mem. Phys. Chim. tom. ii. p. 88.) The time required for pro¬ 
ducing the effect will vary with the degree of light, and the age, texture, 
and peculiarities of the plant. The leaves of French beans, which sprang 
white out of the earth, were observed by Senebier to become green in an 
hour under exposure to an ardent sun; and when etiolated leaves were 
immersed in water, they became green under exposure to sunshine, in 
the same way as in the free atmosphere. (Ibid. p. 78—91.) 
The matter thus acted on by light is contained in the cells of the 
parenchyma; it is green in the leaves, but of different colours in other 
organs of the plant: it is in its nature resinous, and soluble in alcohol. 
By De Candolle, it has been named chromule, from the Greek word sig¬ 
nifying colour. It is the cause of colour in all vegetable surfaces, is common 
to other parts as well as to the leaves, and exhibits different colours in the 
leaves at different periods of the year. (Physiologie Vegetale , t. i. p. 321.) 
In addition to this chromule, there is another matter in the leaves and 
flowers, which, when extracted by water, exhibits a red colour on the ad¬ 
dition of acids, and a yellow or green one on the addition of alkalies. 
This matter, or “ colourable principle,” has been named chromogen by Dr. 
Hope, the distinguished professor of chemistry in this university, in a 
memoir on the 44 Coloured and Colourable Matters in the Leaves and Flowers 
