This change of the sugar of the sap into woody fibre is observable more or less in all plants. When they are shooting fastest the sugar is most abundant; not, however, in those parts which are growing, but in those which convey the sap to the growing parts. Thus the sugar of the ascending sap of the maple and the alder disappears in the leaf and in the extremities of the twig; thus the sugar-cane _sweetens_ only a certain distance above the ground, up to where the new growth is proceeding; and thus also the young beet and turnip abound most in sugar, while in all these plants the sweet principle diminishes as the year’s growth draws nearer to a close. In the ripening of the ear also, the sweet taste, at first so perceptible, gradually diminishes and finally disappears; the sugar of the sap is here changed into the _starch_ of the grain, which, as above described, is afterwards destined, when the grain begins to sprout, to be reconverted into sugar for the nourishment of the rising germ. In the ripening of fruits a different series of changes presents itself. The fruit is first tasteless, then becomes sour, and at last sweet. In this case the acid of the unripe is changed into the sugar of the ripened fruit. The substance of plants,—their solid parts that is—consist chiefly of _woody fibre_, the name given to the fibrous substance, of which wood evidently consists. It is interesting to inquire how this substance can be formed from the compounds, carbonic acid and water, of which the food of plants in great measure consists. Nor is it difficult to find an answer. It will be recollected that the leaf drinks in carbonic acid from the air, and delivers back its oxygen, retaining only its carbon. It is also known that water abounds in the sap. Hence carbon and water are thus abundantly present in the pores or vessels of the green leaf. Now, woody fibre _consists only of carbon and water_ chemically combined together,—100 lbs. of dry woody fibre consisting of 50 lbs. of carbon and 50 lbs. of water. It is easy, therefore, to see how, when the carbon and water meet in the leaf, woody fibre may be produced by their mutual combination. If, again, we inquire how this important principle of plants may be formed from the other substances, which enter by their roots, from the ulmic acid, for example, the answer is equally ready. This acid also consists of carbon and water only, 50 lbs. of carbon with 37½ of water forming ulmic acid, so that when it is introduced into the sap of the plant, all the materials are present from which the woody fibre may be produced. Nor is it more difficult to see how starch may be converted into sugar, and this again into woody fibre; or how, again, sugar may be converted into starch in the ear of corn, or woody fibre into sugar during the ripening of the winter pear after its removal from the tree. _Any one of these substances may be represented by carbon and water only._ Thus,—