When deprived of light, plants are pale, lax and dropsical; restored to it, they recover their colour, consistency and odour. If a plant be placed in a cellar, into which is admitted a small portion of light through a window or cranny, thither the plant directs its growth, and even acquires an unnatural length in its attempt to reach it.[4] These facts admitted, no one can doubt the agency of light in vegetation; but in relation to this agency, various opinions exist; one, that light enters vegetable matter, and combines with it; another, that it makes no part either of the vegetable or of its aliment, but directly influences substances which are alimentary;[5] and a third, that besides the last effect, it stimulates the organs of plants to the exercise of their natural functions.[6] [4] It is by a knowledge of this fact, that gardeners bleach chicony and cellery, &c. [5] See Fourcroy, vol. viii. [6] See Chaptal on vegetation. Without doing more than state these opinions, we proceed to offer the results of many experiments on this subject. 1st. That _in the dark_, no oxygen is produced, nor any carbonic acid absorbed; on the contrary, oxygen is absorbed and carbonic acid produced. 2d. That plants exposed to _light_, produce oxygen gas in water. 3d. That _light_ is essential to vegetable transpiration; as this process never takes place during the _night_, but is copious during the _day_; and, 4th. That plants raised _in the dark_, abound in watery and saccharine juices--but are deficient in woody fibre, oil, and resins; whence it is concluded, that saccharine compounds are formed in the _night_, and oil, resins, &c. in the _day_. When the weather is at or below the freezing point, the sap of plants remains suspended and hardened in the albumum;[7] but on the application of _heat_, whether naturally or artificially excited, this sap is rendered fluid, is put into motion, and the buds begin to swell. Under the same impulse, through the medium of the earth, the roots open their pores, receive nutritive juices, and carry them to the heart of the plant. The leaves, being now developed, begin and continue the exercise of their functions, till winter again, in the economy of nature, suspends the operations of the machine. Nor is its action confined to the circulation of vegetable juices. Without vapour (its legitimate offspring) the fountain and the shower would be unknown--nor would the great processes of animal and vegetable fermentation and decomposition go on. Without rain or other means ameliorating the soil, what would be the aspect of the globe? what the state of vegetation? what the situation of man? [7] Knight's Observations, &c.