The latter group of this large division of lichens comprises the genera allied to Usnea, which are erect, centripetal lichens, that is to say, their body or thallus is an upright and generally cylindrical stem, in the centre of which the layer of marrow, the origin of fructification, is condensed, and the open discs, or hymenia, are in connection with it, whether they be situated at the ends of the branches, or on the surface of the upright stems. The Usneæ have three forms, the pendulous, the inflated and erect, and the branched or shrubby. The Evernia jubata is an example of the first, and remarkable for its long, pendulous, cylindrical branches hanging down like bunches of bluish grey hair. It is often attached to the branches of aged larch trees, and is exceedingly picturesque. With the exception of Evernia vulpina and flavicans, which are brightly coloured, all the rest of the genus have dull tints. The genus Ramalina has the dull tints of the Evernia, and some species approach very nearly in form to that genus. The Ramalina polymorphum and R. scopulorum abound in dyes, while the R. calicaris is rich in gluten. Lichens have lost much of their importance since the discovery of the coal tar colours; nevertheless they afford dyes still in use. In many species the dying principle is colourless like aniline; but it yields the most brilliant colours by means of alkalies. The Parmelia parietina, common on walls and the roofs of houses, gives the chrysophonic acid, a colourless liquid which becomes bright yellow when treated with an alkali; and in like manner the vulpinic acid, given by Evernia vulpina, gives a brown dye. Notwithstanding the quiet greyish green tints of the Rocella fuciformis and R. tinctoria, no lichens are richer in the purple substance known as orchil, from which, by means of soda or potash, the valuable blue substance litmus is manufactured, so important as a test for acidity. Many of the small moss-like lichens yield orchil, but none in such abundance as the R. fuciformis, which grows in Madeira, Angola, Madagascar, and South America. Oxalic and usnic acids are produced by lichens; indeed the usnic acid combined with green and yellow resins seems to be more or less a constituent of various lichens. It is evident that the colour of the dyes is altogether independent of the colour of the lichen from whence they are obtained. These aërial plants have a marvellous power of decomposing the light, and adorning themselves in brilliant hues. In our own country, where the colouring is often cold from the excess of verdure, the lichen affords a happy relief by giving a little warmth to the landscape. Even at Rome, where nature is so gorgeously coloured, the ruins owe much of their picturesque beauty to the red, white, and golden lichens with which they are clothed.