Many garden-varieties, the stability of which under ordinary circumstances is doubtful, because of their being sown too close to other varieties of the same species, have been tested in [159] respect to their stability by different writers and at different times. In doing this it is plain that it is very essential to be sure of the purity of the seed. Specimens must be grown in positions isolated from their allies, and if possible be pollinated artificially with the exclusion of the visits of insects. This may be done in different ways. If it is a rare species, not cultivated in the neighborhood, it is often sufficient to make sure of this fact. Pollen may be conveyed by bees from distances of some ten or twenty meters, or in rare cases from some hundred meters and more, but a greater distance is ordinarily sufficient for isolation. If the flowers fertilize themselves, as is more often the case than is generally supposed, or if it is easy to pollinate them artificially, with their own pollen or in small groups of similar individuals, the best way is to isolate them by means of close coverings. When flowering, the plants are as a rule too large to be put under bell-glasses, and moreover such coverings would keep the air moist, and cause the flower-buds to be thrown off. The best coverings are of netting, or of canvas of sufficiently wide mesh, although after a long experience I greatly prefer cages of fine iron-wire, which are put around and over the whole plant or group of plants, and fastened securely and tightly to the ground.