494 Plant Case for growing Plants 



The great influence which light thus exerts on the colour 

 and properties of plants must be regarded as altogether local in 

 its operation, affecting only those parts to which it has free 

 access ; and, accordingly, the green colour, and other properties 

 to which light gives rise, may be again obliterated by the simple 

 exclusion of that powerful agent. " Thus, if a portion of a 

 growing fruit," says M. Senebier, " be covered with a piece of 

 tin foil, the uncovered portion may become perfectly red, whilst 

 the covered part exhibits only a pale or yellowish hue ; or 

 grapes, which would have acquired a violet colour under a full 

 exposure to light, take on a greyish hue if enclosed in black 

 paper. Those leaves, too, which may only partially cover grow- 

 ing fruit, and thereby intercept the sun's rays, delineate, as it 

 were, on the fruit beneath, the limits they set to its action. (Mem. 

 Phys. Chimiques, torn. iii. p. 146.) In this manner, apples or 

 other fruits may be marked with the impressions of leaves arti- 

 ficially glued on them; and fruits, so marked, it is said, are often 

 exposed for sale in the bazaars of Persia. In North America, 

 the operation of light, 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 some- 

 times 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, dur- 

 ing 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 sum- 

 mer 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 in- 

 stances, 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 mul- 

 tiply, extend, and meet, till the whole exhibits a green colour. 

 This progressive coloration is effected exteriorly by the action 

 of light, and is independent 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 



