An _arboretum_ is a collection of trees and shrubs, in which all the different species of natural order are planted together; and it does not necessarily imply that these orders should be arranged in any particular manner; as indeed botanists are not agreed as to how they should be placed, Jussieu having adopted one plan, De Candolle another, and Dr. Lindley another. The only essential point is, that all the plants which agree sufficiently to form an order should be placed together; as, for example, all the Coníferæ or cone-bearing trees, such as the pines and firs, the cedars, the cypresses, and the junipers; and all the Cupulíferæ or nut-bearing trees, such as the oak, the chestnut, the beech, and the hazel. Thus in your grounds, where there is a valley, it may be formed into what is called an American ground, consisting of the shrubs allied to the heath family, and planted with rhododendrons, azaleas, kalmias, and other similar plants. A glen may be planted with pines and firs, so as to form it into a kind of pinetum; and an open space among trees may be turned into a rose-garden or rosery. In this way most of the exotic trees and shrubs that will stand the air in English gardens may be planted in groups, without going to much expense, and with less formal appearance than that of a regular arboretum. Besides, the beautiful colours which some of the exotic trees will take in autumn give them a splendid appearance, when they are backed by other trees with verdant foliage. The _rose-garden_ will be a very beautiful object if skilfully arranged; and, by thus forming your park into a succession of scenes, you will not only increase its beauty in the eyes of strangers, but add very materially to your own enjoyments by increasing the objects of interest within your reach. I do not suppose you will have any occasion to renovate the _turf_ in any part of your park; but, if you should, the best way is to get a mixture of the seeds of several sorts of grasses, such as the fox-tail meadow grass (_Alopecùrus praténsis_), the sweet-scented vernal grass (_Anthoxánthum odoràtum_), the crested dog's-tail grass (_Cynosùrus cristàtus_), and other valuable kinds. If ever you find a patch of grass has become bare, have an iron rake drawn over the ground two or three times, so as to loosen it, and then sow a few seeds of the grasses I have mentioned, when the ground is in a tolerably moist state from rain. This will generally be sufficient; but if it should not, as the seed of the fox-tail meadow grass is very often bad, you have only to add a little seed of the common white clover, and the ground will appear green in a few days. LETTER X. LAYING OUT A KITCHEN-GARDEN.--MAKING GRAVEL WALKS.--BOX EDGINGS.--CROPS OF CULINARY VEGETABLES.--CUCUMBERS, MELONS, AND MUSHROOMS.