Bulbs of all kinds are specialized buds. They are made up of a short and rudimentary axis closely encased in transformed and thickened leaves or bulb-scales. These thickened parts are stored with nutriment which is used during subsequent growth. Bulbs occur only in plants which are accustomed to a long period of inactivity. Many bulbous plants are peculiar to dry and arid regions, where growth is impossible during long periods. A bulb is, therefore, a more or less permanent and compact leaf-bud, usually occupying the base of the stem underground and emitting roots from its lower portion. Bulbs are conveniently divided into two great classes--the scaly, or those composed of narrow and mostly loose scales, as in the lily, and laminate or tunicate, or those composed of more or less continuous and close-fitting layers or plates, as in the onion. [Illustration: Fig. 10. Bulb of Lilium candidum.] Bulbs often break up or divide themselves into two or more nearly equal portions, as in _Lilium candidum_, shown one-third natural size in Fig. 10. The parts may be separated and treated as complete bulbs for purposes of propagation. This division or separation of bulbs proceeds in a different manner in nearly every species, yet it is so obvious that the novice need not be perplexed by it. Almost any breaking apart of these loose bulbs, if only a "heart" or central axis remains in each portion, is successful for purposes of slow multiplication; but when flowers are desired it is usually inadvisable. Bulbous plants multiply most easily by means of _bulbels_--often also called bulbules and offsets--or small bulbs which are borne about a large or mother bulb. In some lilies, as _Lilium candidum_, the bulbels form at the top or crown of the mother bulb, and a circle of roots will be found between them and the bulb; in others, as _L. speciosum_ and _L. auratum_, they form on the lower part of the flower stalk. In some species the bulbels are few and very large, or even single, and they bloom the following year. In such cases the bulb undergoes a progressive movement from year to year after the manner of root-stocks, the bulb of one year forming a more or less distinct one above and beyond it which continues the species, while the old one becomes weak or dies. This method of bulb formation is seen in the cut of _Lilium pardalinum_, Fig. 11. In the hyacinth the bulbels form at the base of the bulb. [Illustration: Fig. 11. Bulb of Lilium pardalinum.]