175 
may be blanched by excluding them from this agent. Lettuces, endive, celery, 
kale, and other vegetables, are prepared for the table by preventing the access of 
light, as in the operations of tying up the leaves, earthing the roots, or covering 
the whole plant with opaque earthen pots. The bleached pallid appearance of 
greenhouse plants which have been kept in situations where the light has not been 
sufficiently admitted, arises, also, from the same cause; while the brilliancy and in¬ 
tensity of the colours of flowers in tropical and alpine countries is owing to the in¬ 
tensity of the light and the clearness of the atmosphere in such situations. Expo¬ 
sure to light, therefore, tends to develop the colours of plants ; but in what way the 
effect is produced is not so evident. Whether it arises from any chemical change 
in the state of oxydation, or from any physical variation in the optical properties of 
the vegetable tissues from their more vigorous growth and nutrition when under 
the stimulus of this powerful and pervading influence, does not seem clear. The 
green parts of plants, especially the leaves, exhale oxygen, as is well known, on 
exposure to the light of the sun ; while the coloured parts, such as the flowers, 
more frequently exhale hydrogen and azote. By the action of alkalies, also, the 
red colour of many flowers becomes, in succession, blue, green, and ultimately 
even yellow—a change which may possibly be owing to their acting as deoxydiz- 
ing principles. The change in the blossoms of the Myosotis versicolor is from 
yellow to blue, and ultimately to faded purple or red—that is under exposure to 
the light, which, as it induces the exhalation of hydrogen and azote from the 
coloured parts, tends, therefore to the accumulation of oxygen in the same parts, 
the yellow passes successively into blue and a faded purple or pink. I have, how¬ 
ever, never observed any approach to the intermediate stage of green between the 
yellow and the blue flowerets of M. versicolor. The investigation of the 
causes to which the colours of flowers are owing is very important, both in relation 
to vegetable physiology and to optics; and this little plant seems well calculated, 
when submitted to judicious experiments, to afford valuable information to the in¬ 
quirer into these interesting arcana of natural and physical science. 
Boa Constrictors. —A recent traveller in South America, journeying from 
Lima to Vara, in the Brazils, observed that the inhabitants of the latter place take 
great pleasure in rearing the Boa Constrictor (quere Python Tigris) ; and that 
Mr. Smith, the North American consul, possesses several for the purpose of de¬ 
stroying Rats, with which those parts are terribly infested. These creatures 
sometimes attain the length of eighteen feet, and the colours of their skin are bril¬ 
liant beyond description, particularly after moulting. They have never been 
known to injure any one, and even exhibit local attachment to places and persons. 
