There have been waves of the desire to introduce Natural foliage into Architecture (e.g. in the "Decorated period" of Gothic architecture); but the Artificial elements have always proved too strong, and the two have never mixed. In Architecture, everything has three dimensions; and the artificial foliage is carved with leaves, etc., of a suitable thickness: in Natural foliage the tenuity of leaves, etc., is such that it cannot be reproduced. Even in the architraves round the glorious doors of Florence the natural foliage is not always a success; and where Ghiberti has stopped short in the ductile bronze, it is not probable that the modern carver will succeed in stone. It may therefore be suggested that the close imitation of Natural foliage should be confined to objects of _two_ dimensions, i.e., to plane surfaces and figured materials. This selection of the Elements of Decoration, according to their association, is analogous to the selection made use of by the Poet, from the words and ideas, which are his Materials. It will be observed that, as on a Classic or Heroic subject, the choice is of learned words and classical ideas, and on a Domestic or Pastoral one, simple words and homely similes are used--so, in conjunction with the severe forms of Architecture, the formal character of artificial foliage is suitable; and for decorating Textiles and other movable Accessories, the Natural foliage, with which the earth is clothed and beautified, is appropriate. ENRICHMENT OF SURFACE may be beautiful for one reason; IMITATION OF NATURE is beautiful for another. When imitations of natural foliage are introduced decoratively on a surface, then may it be twice beautiful--first, in the _principles_ according to which the distribution is arranged; and secondly, because of the _elements_ which are worked in being beautiful in themselves. Geometrical elements might be so used as to serve the first end, but can never fulfill the second: Storiation fulfills the second; but its increase of interest absorbs the first. This course of Lectures is intended to treat of Natural foliage, leaving Artificial foliage to be dealt with at another opportunity. It is not Historical. The History of the Decorative treatment of Natural foliage, showing its evolution in the past, is a large and interesting theme; but, unless this were accompanied by critical remarks based on given principles, the method might be barren of results. Tradition is not to be undervalued; but the student should be led to Tradition through Principles. It is further intended more especially to apply to the aesthetic use. When natural foliage is used AEsthetically (i.e., decoratively), then the Shape of the surface should govern the Mass shape of the foliage, and there should be Parallelism between them (see Sec. 29). When used Didactically (i.e., symbolically), then the foliage may be treated more freely. Sec. 4.--THE FOUR TREATMENTS.