In considering the various kinds of Roots, we may class them as under [See fuller description in Manual of Botany.]

1° (A) True or tap root (primary).

(B) Adventitious root (secondary).

2° The above distinguished as....

ERIAL. Proceeding through

air, from branches to earth. EPIPHYTES. Roots in, and derives nourishment from, air. PARASITES. Roots growing into the tissues of other plants and thence drawing their support.

1. MONOCOTYLEDONOUS. Given off from one or several points above base of the radical, their structure internally resembles that of the

stem.

2. DICOTYLEDONOUS. Formed by direct prolongation of the radicle.

3. ACOTYLEDONOUS. The "spore" gives off a root at any point of the surface.

THE TRUE ROOT is formed by the direct prolongation of the radicle---cells multiply by division immediately under the layer of cells at the extreme termination, these "vital" cells push away the outer layer and growth then proceeds in the cells behind them, the

hard portion (vessels and wood cells) then forms. It has a species of bark and a cuticle when young. Roots that last only one year, are annual; if two years, biennial; but if longer, perennial. The functions of the root consist not only in fixing the plant firmly to the earth, but also in obtaining nutriment from the soil, and often in storing up organizable matter for its support.

When two fluids of different densities are separated by a permeable medium, two currents will be established---one outwards, called exosomos (exo Gr.. outwards; osmos, impulsion,) and one inwards, called endosmose, (endon, Gr., within,) which will continue until the fluids are of equal density. By means of such currents vegetative action is carried on. The fluid contents of roots are of greater density than the moisture of the soil; an endosmose current is, therefore, established, for which the root-hairs and the newest part of the root, protected by the spongioles, are the medium, accompanied also by a much smaller outward or exosmose current; evaporation continually going on in the exposed portions of the plant, the contained fluids are never reduced to the density of the medium in which they grow, hence the inward current or absorption, does not cease while vital action is unimpaired.

ADVENTITIOUS ROOTs are formed in some plants for the purpose of affording additional support to the stem.

In the ivy they serve the office of tendrils. From the branches of the Banyan they are produced downwards to the soil, in which they fix themselves, and develop true roots.

In the screw pine and mangrove they are developed in a similar manner from the lower parts of the stem, and become rooted in the soil.

THE STEM.

THE ASCENDING AXIS, bearing leaves and also the organs of reproduction; specially distinguished by the presence of leaves with leaf buds in their axils.

I. EXOGENOUS STEMS, (exo, Gr., outward; genano, to produce) or the structure of the stem of outside growers-increase by additions on the outside of the wood-at first cellular; at the end of the first year they present the following parts:

1. PITH.-Cellular tissue---in youth of tree filled with sap---large, in soft, small or obliterated in hard wooded and old trees.

2. WOOD.---Formed from the cambium layer which consists of specially vital cells immediately within the the bark. It comprehends:

a. The Medullary Sheath (spiral vessels) encircling the medulla (marrow) or pith.

b. The Woody Tissue, formed of wood cells, long, overlapping and much thickened by deposit, with occasional pitted vessels.

The wood proper is deposited in zones, generally well defined, one in each year, their number affording a clue to the tree's age. The interior, or heart wood, called also duramen, (from its greater hardness) is black in ebony, and brown in crocus wood, while the external layers of alburnum (albus, white) or sap-wood, are pale colored, soft and permeable.

c. Cambium Layer consists of vital cells containing elaborated nitrogenous sap. Between the external layers of wood, and the bark, a copious semi-fluid mucilage is found in the spring, to which the name of Cambium (cambia, Lat., to change,) has been given.

3. MEDULLARY RAYS.-Layers of tubular cells ("silver grain' of some woods,) connecting the pith and inner bark, thus dividing the woody tissue into segments. They radiate from the centre to circumference, and may be seen in any transverse section of Exogenous stems.

4. BARK.-Outer covering of stem, growing by additions internally; it consists of:

a. Endophloum (endon within) or inner coat; this is the liber or bast tissue of wood cells. These bast cells are arranged variously in the parenchyma of the liber of different barks, (viz. radially, tangentially, &c.)

b. Mesophloum or middle layer; green spongiform, cellular layer. c. Epiphloum, corky or suberous layer; dark, flat, tabular cells, often much developed.

d. Epidermis, or periderm (cuticle) is extended over all parts of the plant, exposed to the atmosphere, except the stigma.

II. ENDOGENOUS STEMS, or the structure of stems increasing by additions to the inside. No distinction of parts, there being no true pith, bark, or regular zones of wood-fibro-vascular bundles, (wood cells, spiral and pitted vessels,) being destributed irregularly throughout the cellular system. On the outside of the trunk there is formed a false bark or rind. Thus the stem of such a plant is limited in its lateral expansion-generally very tall and slender.

III. ACROGENOUS STEMS, or those growing at the summit, peculiar to flowerless plants. In mosses, &c., it is simply cellular; in the higher classes of ferns, it is a cellular mass, with simultaneous vascular bundles, arranged in wavy, plate-like masses, towards the outside; these bundles generally contain scalariform vessels; on the exterior is a rind marked with scars of fallen leaves. These stems grow only from the terminal bud. The leaves are called fronds, and the petioles are called stipes.

SUBTERRANEAN STEMS.

RHIZOME, (rhiza, Gr., a root,) or rootstalk; distinguished from roots by possessing scales (modified leaves,) or buds, (rudimentary leaves.) It is a thick, generally a procumbent sort of stem or root stalk

partly, and sometimes entirely underground; develops roots from its under side and leaves from its upper. Its surface generally bears the scars left by the falling away of old leaves. It has nodes or joints, at which point they produce buds, which, in time, develop into a new plant. The thickened portion contains starch or other nourishing material, upon which the developing plant feeds.

TUBER (Lat. knob,) is a thickened portion of an underground stem or branch, which serves as a depository for starch and other nutritious secretions for the service of the plant; slender branches become enlarged at the growing end, by the accumulation there of this material. Ex. potato.

BULBS are subterranean buds or shortened stems surrounded by by thickened scales-roots from the under surface, directed downwards. The Pseudo-bulbs of orchids, are thickenings of the base of stems, by a deposit of bassorin; they are produced above the surface, and are of a green color, Ex-salep. New bulbs are formed like buds, in the axils of the scales.

CORMS, enlarged, solid, oval stem, or "base of stem" (Lindley) often amylaceous, producing buds on their upper surface and roots below.

(Bulbs, Pseudo bulbs and Corms are confined to endogenous Plants.)

LEAVES.

These are lateral expansions of the cellular tissue of the stem circumference, with veins as a continuation of the vascular system. The cellular system surrounds the fibro-vascular ramifications in two layers; the cells contain chlorophyll and are loosely connected. The epidermis is on both sides. The stomata (breathing pores) chiefly on the under side.

Submerged leaves consist wholly of cellular tissue.

LEAF VARIATION IN:

VENATION. MARGIN. STIPULES,

ARRANGEMENT

Dicotyledons, Reticulated, Indented, often present Compound or

Monocotyledons, Parallel,

Entire, absent,

Acotyledons, Forked, Much cut, absent,

simple,

Simple,

Compound.

PARTS.-Blade; Petiole; Stipule, (sheath and ligule, common in grasses;) Tendril.

FRAMEWORK.—Midrib, rib, Vein and Veinlet.

THE LEAF MAY BE,

Simple, (as Digitalis,) when the blade is of one piece. Compound, (as Conium,) when the blade consists of two or more pieces borne upon a common leaf stalk.

THE ARRANGEMENT MAY BE, Alternate, Opposite or Whorled. FORM.-BROAD AT THE MIDDLE—

Peltate, (shield shape)

Orbicular (circular.)

Oval, (egg shaped, widest in middle)
Elliptical, (long, the two ends alike in width.
Oblong, (twice or thrice as long as broad).
Linear, (long and narrow-same breadth
throughout.)

BROAD AT BASE-Deltoid, (triangular.)

Ovate. (egg shaped, widest at the base.)
Lanceolate, (lance shaped.)

Subulate, (awl shaped.)

Cordate, (heart shaped.)

Reniform, (kidney shaped.)

Hastate (halberd shaped.)

Sagittate, (arrow shaped.)

BROAD AT APEX-Obovate, (reversely ovate, widest at apex.) Oblanceolate, reversely lanceolate, tapering point downwards.

Spathulate, (resembling a spoon).

Obcordate, (reɣersely heart shaped.)