314 
THE LADIES* MAGAZINE OF GARDENING. 
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 chromogen, or the a colourable principle,” is not 
an individual substance, as hitherto supposed; but that there are two dis¬ 
tinct principles, one, which forms the red compound with acids, which he 
names erytlirogen ; 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 possessing an orange 
chromule, and in the corolla of twenty purple flowers, both colourable 
principles were also found. Other parts of flowers, as the calyx, bractem, 
&c. comported themselves 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 accom¬ 
panied, or rather preceded, by the evolution of oxygen gas; whilst under 
every degree of light, flowers always deteriorate the air. 
As the solar light consists of rays possessing very different powers, M. 
Senebier endeavoured to discover to which species of rays the colouration 
of the leaves of plants was to be specially ascribed. Scheele had remarked 
that the violet rays of the prismatic spectrum acted soonest in blackening 
muriate or silver ; a fact confirmed by the experiments of Senebier, who 
extended the same views to the action of light in the colouration of plants. 
He caused young colourless plants to grow in different glass vessels, so 
constructed that the light which fell upon them should first pass through 
fluids of different colours, red, yellow, and violet. At the end of four or 
five weeks, the leaves which had been exposed to red light had a tinge of 
green ; those in the yellow light were at first green, but afterwards 
became yellow; and those in violet light were quite green, and the 
depth of colour increased with their age. [Mem. Phys. tom. ii. p. 55, 
et seq .) The subsequent experiments of Ritter and Wollaston have shown 
that these effects were produced, not by the coloured rays, but by certain 
invisible rays associated with them, and which exist in greatest force at 
and beyond the boundary of the violet extremity of the spectrum. To 
these rays have been assigned the names of the chemical or deoxidating 
