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COMMUNICATIONS, to- 



being given out, and oxygen absorbed. Plants that have been so treated recover their vigour by 

 being brought into light and air. It is to the action of light, heat, and air, that the different 

 parts of plants assume their various tints of colour which are so very pleasing to the eye in the 

 flowers, in the leaves, and in some bractese, as for instance in those of Poinsettia pulcherrima, an 

 elegant plant figured in our last number, and also in the fruits which but for that would be 

 sapid and tasteless. The subject of Colour has for some time engaged the attention of Philoso- 

 phers on the Continent, and a long account of it has been published in De Candolle's Physiologie 

 Vegetale, under the head Chromule, a name given to it after many others, meaning colour to 

 distinguish it, but the reason of the different tints does not appear at present known with 

 accuracy. Its properties have been investigated by Macaire, a distinguished Vegetable Chemist 

 of Geneva, who says that the tissue of the cells of the parenchyma (or pulpy matter which 

 contains the colour) is analogous to lignine, and contains a greater or less portion of wax ; he 

 having deprived a leaf of its cuticle first boiled it in aether, and a kind of gluten and other 

 substances were obtained in small quantities. To procure the chromule he digested a portion of 

 this pulp in rectified alcohol, afterwards boiled it in aether, and after that he well pressed - and 

 washed it. He then filtered and evaporated it, and obtained a substance to all appearance 

 resinous, of a dark green colour, and which no doubt is the substance that gives the colour to the 

 leaf. By means of boiling water, a brown extractive matter was obtained which is not crystal- 

 lizable, not altered by exposure to the air, but softens when held at the fire, and is decomposed 

 after the manner of vegetable substances, It is insoluble in water, easily soluble in alcohol 

 aether, the fixed and essential oils, also in solutions of potass and soda, in concentrated sulphuric 

 and acetic acids. Its composition is hydrogen and carbon, with a small portion of oxygen, but it 

 has not as yet received an accurate analysis. The quantity of oxygen in this matter seems 

 various. Senebier a long time ago observed that the coloured leaves of Autumn exhale much 

 oxygen gas. Mr. Macaire experimenting with the green leaves, but which were approaching a 

 change of colour, found that they exhaled oxygen during the day, and inspired it during the 

 night, and this latter function was continued in a few which had changed their colour ; hence he 

 concluded that oxygen fixing itself' on the chromule gives its yellow tint, and when in greater 

 proportions its red one. He has seen the leaves of the Italian poplar yellow in the Autumn, and 

 being treated by the process above given, gave a yellow chromule which differed in nothing from 

 the green, unless by its insolubility in fixed and essential oils. A cold infusion of the same in 

 alkalies became of a beautiful yellow which was soluble in oil. The red chromule of the leaves 

 of Sumac taken in Autumn turn to their natural green by alkalies. On the contrary, acids turn 

 the green chromule first to yellow, thence to red, according to the intensity of their action. Mr. 

 Macaire has besides noticed that the red chromule of the bractese, calices, and also the petals of 

 Salvia splendens presents the same properties as those of the leaves which redden in Autumn. 



The chromule of the yellow flowers is also changed to green by alkalies. We have extracted 

 a great part of this interesting subject from De Candolle's Physiologie Vegetale, a work of 

 considerable talent, for which he received the Gold Medal of the London Horticultural Society. 



It being a work that would greatly benefit Gardeners generally, from the soundness of its 

 views, which are of primary importance ; we are surprised that some person does not undertake 

 an English translation for their benefit, and comprised in one closely printed 8vo volume, sold at 

 a reasonable price. It might then be studied with advantage by every practical gardener. 



