Fig. 756.—Section of an endogenous stem.
(2) The culno, a thin, hollow, and frequently-jointed stem.
(3) The palm or simple stem, seen in tree-ferns and palms. It is marked by the scars of dropped leaves.
(4) The stalk, very common, of a green hue, and its life is limited to a twelve-month as a rule. The so-called “stem” of the hyacinth is not a stem, it is a stalk, or flower-stalk, pushed forth for a temporary purpose.
(5) The ligneous stem is the perfected kind, and an example will be apparent in every tree.
The duration of the stem of a plant is usually the same as of the plant—so we have annuals, biennials, and perennials. The substance of the stem determines its character, so we may have it solid, or soft, hollow, tubular, flexible, rigid, or a tough stem. There are fibrous, herbaceous, and juicy stems. They may be directed uprightly, straight, procumbently, arched, or creeping, above, or underground, climbing, clinging, floating, or twining.
There are many plants with little or no stem deserving the name, as in the onion; and we must all remember when studying botany that it is not the place where a portion of a plant may be found that constitutes it a root or a stem. The form and structure should be studied, and its purpose in creation. So stems may be underground and roots above it. The root and stem, briefly treated of in the foregoing paragraphs, have certain points of resemblance, inasmuch as both consist of a main or trunk line, so to speak, from which branches diverge as “rootlets” and “twigs”; and how beautiful the latter are any one can see in a good photograph of a wintry landscape. But stems have nodes and internodes, and roots have not, and this is the great and apparent difference.
The covering of plant-stems is varied, and many instances of such clothing will occur. We have woolly stems and hairy stems, which develop into thorny ones—for thorns are only strong hairs. Spines and stings and prickles defend the stems, and keep rude hands from meddling. We will now cut the stem and see what it is composed of, and how it looks inside. We have only to cut it across and again perpendicularly to find out a great deal about the interior structure of the stems of branching plants (exogens).
The elder, from which the whistle of our boyish days is fashioned with a penknife, will serve any lad for an illustration. Inside we find what is called “pith,” which is cellular tissue. Round this is fibre, and outside is a skin, or the plant-cuticle. We may remark that the tissues of flowering plants are characteristic of the monocotyledonous and the dicotyledonous plants. Of the former we append an illustration,—a section of palm-stem,—and we find bundles of vascular tissue dispersed apparently at random amongst the cellular tissue of the parenchyma, or cellular tissue. These stems do not grow by the increase of the existing vascular tissue, but by their new production at the circumference, and so they grow in both directions, laterally and uprightly. These plants belong to the Endogens, and if Indian corn be grown we shall have full opportunity to study the formation. In cutting a fern stem we are familiar with the “oak” pattern of the matter it contains. We have few specimens of endogens in England.