86. =Perennials= live and blossom year after year. A perennial herb, in a temperate or cooler climate, usually dies down to the ground at the end of the season's growth. But subterranean portions of stem, charged with buds, survive to renew the development. Shrubs and trees are of course perennial; even the stems and branches above ground live on and grow year after year. 87. There are all gradations between annuals and biennials, and between these and perennials, as also between herbs and shrubs; and the distinction between shrubs and trees is quite arbitrary. There are perennial herbs and even shrubs of warm climates which are annuals when raised in a climate which has a winter,--being destroyed by frost. The Castor-oil plant is an example. There are perennial herbs of which only small portions survive, as off-shoots, or, in the Potato, as tubers, etc. Section VI. STEMS. 88. =The Stem= is the axis of the plant, the part which bears all the other organs. Branches are secondary stems, that is, stems growing out of stems. The stem at the very beginning produces roots, in most plants a single root from the base of the embryo-stem, or caulicle. As this root becomes a _descending axis_, so the stem, which grows in the opposite direction is called the _ascending axis_. Rising out of the soil, the stem bears leaves; and leaf-bearing is the particular characteristic of the stem. But there are forms of stems that remain underground, or make a part of their growth there. These do not bear leaves, in the common sense; yet they bear rudiments of leaves, or what answers to leaves, although not in the form of foliage. The so-called stemless or _acaulescent_ plants are those which bear no obvious stem (_caulis_) above ground, but only flower-stalks, and the like. 89. =Stems above ground=, through differences in duration, texture, and size, form herbs, shrubs, trees, etc., or in other terms are _Herbaceous_, dying down to the ground every year, or after blossoming. _Suffrutescent_, slightly woody below, there surviving from year to year. _Suffruticose_ or _Frutescent_, when low stems are decidedly woody below, but herbaceous above. _Fruticose_ or _Shrubby_, woody, living from year to year, and of considerable size,--not, however, more than three or four times the height of a man. _Arborescent_, when tree-like in appearance or mode of growth, or approaching a tree in size. _Arboreous_, when forming a proper tree-trunk. 90. As to direction taken in growing, stems may, instead of growing upright or erect, be _Diffuse_, that is, loosely spreading in all directions. _Declined_, when turned or bending over to one side. _Decumbent_, reclining on the ground, as if too weak to stand. _Assurgent_ or _Ascending_, rising obliquely upwards. _Procumbent_ or _Prostrate_, lying flat on the ground from the first. _Creeping_ or _Repent_, prostrate on or just beneath the ground, and striking root, as does the White Clover, the Partridge-berry, etc.