The student afield, with no equipment save a penknife and a pocket-lens, and with mayhap but a limited stock of patience, is content to know that this woody thread is a fibro-vascular bundle, and that its important parts are wood-vessels, bast-tubes, and tough fibres, which give strength and support to the whole affair. Further support is given to the fibro-vascular bundle of a monocotyledon or a fern by a bundle-sheath made of corky cells or of cells with very thick walls.
The kin of the rose, too, form fibro-vascular bundles, and tough ones at that. When, in ridding the lawn of an intrusive plantain, one gives a pull to the tuft of leaves they are apt to tear away, leaving whitish strings dangling from the broken surfaces. These are the fibro-vascular bundles of the leaf-stem, and so are the strings, which must be removed from imperfectly-frosted table-celery.
In the "wood" of a bundle are included all those vessels through which fluids ascend from the roots toward the leaves.
The walls of some of these are queerly pitted, and those of others are beautifully marked with raised rings or spirals.
When the plant is growing actively the largest vessels generally contain but a film of fluid covering their walls, while the rest of the space within them is filled with air.
The tubes, which are the most important part of the bast, have thin walls with delicate tracery, and are the route by which fluids descend from the leaves toward the roots.
The water which a growing plant absorbs from the soil holds in solution many mineral and chemical substances. This liquid is "crude sap," and it is the material upon which three magicians work together. The green coloring-matter in the leaves, the sunlight falling upon them, and the carbon dioxide in the air about them, are the efficient trio.
And the result of their subtle alchemy is "digested sap," which moves downward from the leaves into all the growing parts of the plant, travelling always through the bast.
When Nature is about to make a new fibro-vas-cular bundle, in lily-kin or in rose-kin, a little of the cellular substance of the growing stem experiences a change of character, and becomes set aside, as it were, for new and higher uses. Each cell in such a cluster divides lengthwise into two, which again divide, each into two. This young tissue instinct with formative life is "procambium." After a little while the cells on one side of it lengthen, their walls grow thicker, and on them appear the annular and spiral markings characteristic of the first-formed wood-vessels. On the other side of the procambium, meantime, bast-tubes are taking shape and office.
In the palmetto-trunk, in the corn-stalk, and in the stems of most lilies the cells of the procambium soon cease to multiply, and they all become altered over into wood or bast before the close of the growing season.
Thus Nature comes to the end of her material, and the growth of the bundle ceases perforce. Fibro - vascular bundles of this nature, which can grow "just so much and no more," are called "closed," and are very general among monocotyledons. They are shut off from one another by masses of pith, and there is not, at any season, a continuous ring of young tissue running around the stem. So it is only in a few exceptional cases that the monocotyledonous stem grows steadily thicker with age.
The corn at six weeks old is more sturdy than when it first rises above ground, but this is mainly because the second joint of the stem is larger than the first, and the third larger than the second. So if we push away the earth from the base of growing corn we find that the portion closest to the ground is more slender than the portion above. But the increase in the diameter of the corn-stalk, lily-stem, or palmetto-trunk is entirely limited to the earliest period of growth. Some of the oldest palmettos in Florida are noticeably slender.
Among all the lily's many kin there is but one native plant which grows stouter as it grows old. This is the yucca or bear-grass of the Southern States, which is interesting to botanists as a connecting link between two great classes of plants, for it is a monocotyledon in everything except its mode of growth, and in that it resembles the dicotyledons. For the kin of the rose grow stouter and sturdier with every year of life.
The main stem and older branches of a rosebush have a tough bark, which peels off readily in strips. If we examine this carefully we find that it consists of two portions. The outer layer is thin and colorless, and in an old rose-bush it is dry and apparently half dead. Inside this is a layer of green bark, full of sap and vitality.
Beneath this lies the wood - a hollow cylinder, enclosing a light porous substance - the pith. This marked division of the stem into concentric rings of bark, wood, and pith is found only in the dicotyledons.
In a very young plant of the rose's kin this distinction is not yet apparent.
Indeed, a cross-section of any very young flowering-plant shows a stem-tissue alike in every part. In the lily's kin it all behaves alike, for any cluster of cells anywhere in the young stalk may turn into procambium.
But when a young plant of the rose's kin is about to acquire fibro - vascular bundles the little clusters of cells which become instinct with constructive life lie just beneath the surface of the stem.
Next spring's bundles will develop in the spaces between those of last spring, and by time the stem is four or five years old it has a ring of bast running all around it and a ring of wood within. Between these, in spring, there is a circle of cells which are actively at work building up new tissue.
When we peel the bark off a spring bough we break these forming cells, and the jelly which fills them escapes, moistening the wood, and our destructive fingers also.
During this season of vigorous growth the fibro-vascular bundle of a dicotyledon consists, broadly speaking, of three parts - the wood, the bast, and the generative tissue, full of sap and vitality, which lies between them.