What change does carbonic acid undergo after entering the plant? In what parts of the plant, and under what influence, is carbonic acid decomposed?] The roots of plants terminate at their ends in minute spongioles, or mouths for the absorption of fluids containing nutriment. In these fluids there exist greater or less quantities of carbonic acid, and a considerable amount of this gas enters into the circulation of the plants and is carried to those parts where it is required for decomposition. Plants, under favorable circumstances, may thus obtain about one-third of their carbon. Carbonic acid, it will be recollected, consists of _carbon and oxygen_, while it supplies only _carbon_ to the plant. It is therefore necessary that it be divided, or decomposed, and that the carbon be retained while the oxygen is sent off again into the atmosphere, to reperform its office of uniting with carbon. This decomposition takes place in the _green_ parts of plants and only under the influence of daylight. It is not necessary that the sun shine directly on the leaf or green shoot, but this causes a _more rapid_ decomposition of carbonic acid, and consequently we find that plants which are well exposed to the sun's rays make the most rapid growth. [Explain the condition of different latitudes. Does the proportion of carbonic acid in the atmosphere remain about the same?] The fact that light is essential to vegetation explains the conditions of different latitudes, which, so far as the assimilation of carbon is concerned, are much the same. At the Equator the days are but about twelve hours long. Still, as the growth of plants is extended over eight or nine months of the year, the duration of daylight is sufficient for the requirements of a luxuriant vegetation. At the Poles, on the contrary, the summer is but two or three months long; here, however, it is daylight all summer, and plants from continual growth develop themselves in that short time.