Page:An introduction to physiological and systematical botany (1st edition).djvu/243

 Rh of the animal and vegetable kingdoms on each other, which had never been suspected before his time. Comparative experiments upon the lower tribes of these kingdoms have not yet been made, but they would probably afford us a new test for distinguishing them. The air so copiously purified by a Conferva, one of the most inferior in the scale of plants, may be very extensively useful to the innumerable tribes of animated beings which inhabit the same waters. The abundant air-bubbles which have long ago given even a botanical name to one supposed species, Conferva bullosa, are probably a source of life and health to whole nations of aquatic insects, worms and polypes, whenever the sun shines.

In the dark, plants give out carbon and absorb oxygen: but the proportion of the latter is small, compared to what they exhale by day, as must likewise be the proportion of carbon given out; else the quantity of the latter added to their substance would be but trifling, especially in those climates, where the proportion of day to night is nearly equal, and which, notwithstanding, we know