Sheffield Botanic Garden. - This garden occupies 18 acres, on a bank with a varied surface sloping considerably to the south. It has been laid out by Mr. Marnock, in a manner which, as far as we had time to examine it, appeared perfectly satisfactory, and decidedly in better taste than any garden of the kind which we have yet seen. In short, there is nothing in it which we could wish to alter. The arboretum and fruticetum is so arranged as to display every specimen tree and shrub from the walks; and, when these specimens shall have been a few years grown, their picturesque effect will be such that no other trees or shrubs but the specimens will be required. At present the named kinds, which are finally to remain, are mixed with other sorts to shelter them; but these shelter plants are few, and what is of more consequence, from being chiefly of one kind in one place, they do not even now drown, so to speak, the effect of the plants which are finally to remain. The shelter plans are chiefly Ontario poplar, a species of poplar that has creeping roots; and which, like all trees having creeping roots, may be safely transplanted even when of a considerable size. Hence these nurse plants, as. they are thinned out, are sold by Mr. Marnock to persons in the neighbourhood making plantations, or laying out small places. The nurse plants employed in botanic gardens hitherto, and more especially in the Chiswick Garden, have been a mixture of various kinds (see p. 350.), which distract the eye, and puzzle it to find out the specimen plants which are ultimately to remain; but when the nurses are all of one species, though a general sameness is produced as well as in the other case, yet, when examined in detail, this sameness is of a more simple kind, and one which affords greater facilities for discovering the specimen plants. As a general principle, therefore, where nurse plants are to be introduced into a scientific or ornamental plantation, one kind ought always to prevail in one place. The Ontario poplar seems very judiciously chosen as a nurse plant for a scientific garden, because it comes early into leaf, and does not grow faster than the average of trees: the black Italian poplar grows much too fast, as does the larch; but the mountain ash, the wild sorb, the common Sycamore, the lime, and similar trees, are quite suitable. The greater part of the specimen trees in the Sheffield Garden are planted in masses which will finally be open groves; but all the more hardy and vigorous-growing sorts, and many of the shrubs, stand singly on the lawn. The masses are dug; and the direction of the margins indicating the termination of one and the commencement of another genus is such, that at the termination of each genus an angle (as in fig. 121.) is formed in the outline of the mass. This angle always prepares the observer for a change of genus.