'without fresh Supplies of Water and Air. 495 



French beans, which sprang white out of the earth, were ob- 

 served 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 year (Physiologic vegetale, t. 1. 

 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 addition of acids, and a yellow or 

 green one on the addition of alkalies. This matter, or " colour- 

 able principle," has been named chromogen by Dr. Hope, the 

 distinguished professor of chemistry in this university, in a 

 memoir on the " Coloured and Colourable Matters in the Leaves 

 and Flowers of Plants," read to the Royal Society of Edinburgh 

 in 1837. From numerous experiments, made on various leaves 

 and flowers, Dr. Hope was led to the conclusion, that chromo- 

 gen, or the " colourable principle," is not an individual sub- 

 stance, as hitherto supposed ; but that there are two distinct 

 principles, one, which forms the red compound with acids, 

 which he names erythrogen ; and another, which affords a yellow 

 compound with alkalies, which he calls xanthogen. These 

 principles exist sometimes separately and sometimes together in 

 different plants, or in different parts of the same plant. All 

 green leaves, all white and all yellow flowers, and white fruits, 

 contain xanthogen alone ; whilst in red and blue flowers, and in 

 the leaves of a few plants which exhibit the former of these 

 tints, these two principles occur together. In ten flowers pos- 

 sessing an orange chromule, and in the corolla of twenty purple 

 flowers, both colourable principles were also found. Other 

 parts of flowers, as the calyx, bracteae, &c, comported them- 

 selves as the corresponding coloured chromules of the flowers 

 do. Litmus presented the solitary example of a substance 

 abounding largely in erythrogen, but containing no xanthogen. 

 Light, adds Dr. Hope, was indispensable for the production of 

 the green chromule of leaves ; but not for the formation of some 

 of the finest tints of flowers and fruits, if essential for any : 

 differences connected, probably, with the fact, that the formation 

 of the green colour in leaves is always accompanied, or rather 

 preceded, by the evolution of oxygen gas; whilst, under every 

 degree of light, flowers always deteriorate the air. 



