51 
ON THE VETERINARY MATERIA MEDICA. 
the leaves have been, not inappropriately, called the lungs of the 
plant. The nature of the process, however, varies considerably 
with circumstances. During the modified light of a cloudy day, 
and to much greater extent during the night, the leaves perform 
the precise office of the lungs. A considerable portion of carbon, 
with which the venous blood was overloaded, is given out: it 
unites with the oxygen of the air, and carbonic acid gas is form¬ 
ed. (A vessel was here produced under which some leaves had 
been placed. A light was immediately extinguished.) 
When, however, a healthy plant is exposed to the direct rays 
of the sun, a different process is carried on. Stimulated by the 
lioht. it is enabled to derive nutriment from the surrounding air. 
It decomposes the carbonic acid. It attracts to itself the carbon, 
and the oxygen is left. Others say, and have endeavoured expe¬ 
rimentally to prove, that it gives out oxygen. The lectuiei in¬ 
clined to the former opinion; but whichever was the modus ope- 
randi, oxygen was eliminated. (Another receiver was now pro¬ 
duced, in which a healthy young plant had been exposed to the 
direct rays of the sun. The taper burned with considerably in¬ 
creased brilliancy.) The influence of plants in purifying or dete¬ 
riorating the air depended on the presence and agency of the 
solar li«ht. In the open air little danger was ever to be appie- 
hended'; but plants kept in a close room, and particularly m a 
bed-room, could not contribute to health. 
Lioht does not seem to influence the growth of plants, but is 
intimately connected with their colour and properties. Tansy at 
the bottom of a mine loses its colour and smell. Celery buried 
deep in the mould loses its acrimony, and becomes palatable and 
white. Thus the theory of bleaching our herbs for salads. 
When a plant has been long placed in perfect darkness, and 
has suffered in its colour and taste, if it be brought to the light, 
it absorbs carbon, or produces oxygen most rapidly. A piece of 
celer\ r , placed in the sunshine under a receiver, had produced a 
considerable quantity of oxygen. 
The flower is attached to the plant by the stalk, botamcally 
named the peduncle. At the extremity of the peduncle is the 
califx, covering the flower before it unfolded, and supporting and 
defending it afterwards. The flower or corolla is divided into 
leaves or °petals : some of them are delicately and beautifully co¬ 
loured, and exhale the most fragrant perfume. The colour and 
smell contribute to the enjoyment of human beings, but they weie, 
perhaps, principally designed to attract wandering insects, who 
materially assist in completing the work of reproduction. Both the 
colour and smell arc much influenced by solar light. I he nature 
