Lichen and moss grow on the mortar wherever it has become slightly disintegrated; and if any mould, however minute, by any means accumulates between the slates, there, too, they spring up, and even on the slates themselves. Tiles are often coloured yellow by such growths. On some old roofs, which have decayed, and upon which detritus has accumulated, wallflowers may be found; and the house-leek takes capricious root where it fancies. The stonecrop is the finest of roof-plants, sometimes forming a broad patch of brilliant yellow. Birds carry up seeds and grains, and these germinate in moist thatch. Groundsel, for instance, and stray stalks of wheat, thin and drooping for lack of soil, are sometimes seen there, besides grasses. Ivy is familiar as a roof-creeper. Some ferns and the pennywort will grow on the wall close to the roof. A correspondent tells me that in Wales he found a cottage perfectly roofed with fern--it grew so thickly as to conceal the roof. Had a painter put this in a picture, many would have exclaimed: "How fanciful! He must have made it up; it could never have grown like that!" Not long after receiving my correspondent's kind letter, I chanced to find a roof near London upon which the same fern was growing in lines along the tiles. It grew plentifully, but was not in so flourishing a condition as that found in Wales. Painters are sometimes accused of calling upon their imagination when they are really depicting fact, for the ways of nature vary very much in different localities, and that which may seem impossible in one place is common enough in another. Where will not ferns grow? We saw one attached to the under-side of a glass coal-hole cover; its green could be seen through the thick glass on which people stepped daily.