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ONCE A WEEK. [Jolt 23, 1864.
when plants are placed in such circumstances that they cannot decompose carbonic acid, so that they cease to exhale oxygen, they become etiolated, or turn white. Were it possible, then, for the darkness of night to be prolonged, chlorophyl, or leaf-green, would disappear from creation. The grass of our meadows and the foliage of plants would turn white, and the whole of their vegetative energy would be expended in pushing forth weak, watery, and etiolated shoots and leaves. The same remark applies to the resins, volatile oils, wax, and other vegetable products which plants elaborate from the sap. Sunlight, or at any rate diffused daylight, is necessary to their formation, and the process stops during the night.
But not only leaves and blades of grass, but flowers are coloured by the sun. The beautiful cluster of leaves popularly called the flower, is only the ordinary green leaves of the plant carried forward to a more advanced stage of organic metamorphosis. The petals of flowers are usually greenish whilst folded together in the bud, and they only change their colour and obtain those charming and radiant hues by which they are distinguished, when they open and become exposed to the warm bright rays of the sun.
Nature shows us this change of colour as if for purposes of instruction, with unusual rapidity in some flowers. The following flowers go through periodic changes of colour:—
Hibiscus mutabilis. This is a malvaceous plant, a native of the East Indies, which has been properly called a vegetable chameleon. In the morning its flowers are white, at midday pink, and in the evening they shine with the colour of the rose.
Gladiolus versicolor This is a species of sword lily, and a native of the Cape of Good Hope. In the morning its flowers are brown outside and yellow inside, and in the evening a clear blue. During the night the blue colour disappears, and in the morning has changed back to brown, and so for eight days this change of colour takes place. In Brunsfelsia, a plant belonging to the natural order Solanacœ, the flower when it first opens is white, afterwards it changes to a straw colour. All the Franciscea species (another Solanaceous plant) begin the day with blue violet flowers and end it with lilac and white. Among our native plants the Lithospermum purpueocaruleum, or gromwell, changes the colour of its flowers from blue to purple as the day advances.
These diurnal changes in the colour of the flowers can only result from chemical changes in the coloured fluid or semi-fluid matters which fill the cells of the petals, the result of the variability of the amount of heat and light received from the sun during the day. It may be called solar chemistry, a science at present but little understood.
In the inorganic kingdom of nature we have many proofs of the influence of the sun's rays in effecting chemical changes. Chloride of gold dissolved in water will be precipitated in the form of gold leaf, if the solution be exposed to the sun's light. Paper sponged with a solution of chloride of silver darkens slowly in diffused daylight, but darkens in two or three minutes only by exposure in sunshine. So also the daguerreotypist succeeds better in the morning when the sun shines brightly than on a cloudy day, or in the evening, in the exercise of his art.
The odour of flowers is also affected by these daily changes of temperature, light, and other conditions of plant life. Some flowers, such as the Leucojum, or snowflake, and the Œnothera, or evening primrose, are more fragrant in the evening than they are in the morning. Each flower, in fact, has its own time of emitting its fragrance. The fragrance, as well as the colour of some flowers, is changeable. There is a species of Cestrum in Mexico, the odour of which changes in a most remarkable manner. It is called by the Mexicans "Angel de dia y punta de noche," or an angel by day and a dung-maid by night; because it gives forth a delightful smell by day and stinks at night, resuming its agreeable odour again in the morning. It is impossible to avoid the conclusion that the sunlight plays an important part in effecting this change of odour.
Perspiration.—If a plant with foliage is placed under a glass vessel and exposed to the sun, the sides of the vessel are soon covered with moisture, which is produced by the condensation of the insensible perspiration from the plant. This varies during the different hours of the day, depending on the dryness or moisture of the atmosphere, and the amount of light received from the sun. In bright sunshine, provided the atmosphere is dry, plants perspire most; in weak, diffused daylight least, and in darkness not at all. Morning and noon are, therefore, most favourable to perspiration; it diminishes in the afternoon, and ceases at night. It depends also on the number of leaf-pores and the amount of exposed leaf-surface. Compound leaves perspire more than simple leaves, evergreens less than trees with deciduous foliage.
A remarkable example of the influence of light on the juices of plants is mentioned by Liebig in the Cacalia ficoides. During the hours of darkness this plant, like others, assimi-