216 REMARKS ON GROWING PLANTS IN GLASS CASES. 



in colouring the leaves of plants, is sometimes exhibited on a great 

 scale, and 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 coloration in an etiolated leaf, when exposed to 

 sunshine, 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 

 coloration is effected exteriorly by the action of light, and is indepen- 

 dent of the internal vegetation of the organ. — {Mem. Phys. Chim., 

 torn. ii. p. 88.) The time required for producing 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, signifying 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 



