REMARKS ON GROWING PLANTS IN GLASS CASES. 217 



year. — (Pliysiologie Vegetate, torn. i. 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 " colourable 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 en various leaves and 

 flowers, Dr. Hope was led to the conclusion that chromogen, or the 

 " colourable principle," is not an individual substance, 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 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, bracteae, &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 con- 

 taining no xanthogen. Light, adds Dr. Hope, was indispensable for 

 the production of the green chromule of leaves, but not for the forma- 

 tion of some of the finest tints of flowers and fruits, if essential for 

 any : differences connected, probably, with the fact, that the forma- 

 tion 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. 



As the solar light consists of rays possessing very different powers, 

 M. Senebier endeavoured to discover to which species of rays the 

 coloration 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 of silver, a fact confirmed by the expe- 

 riments of Senebier, who extended the same views to the action of 



