On Molecular and Microscopic Science, Volume 1 (of 2)

by Mary Somerville

The generating layer of cambium, in which all the phenomena of growth takes place, is a semi-fluid mucilaginous substance, which comes between the liber and the wood. It is most abundant in the spring, and is the origin of all horizontal growth. This mucilage is really made up of a vast multitude of cells, with cell-walls as delicate as those of a soap bubble, which gradually undergo transformation into woody fibre, laticiferous ducts, spirals, &c., thence called the cambium zone. The whole of this matter spontaneously divides into two parts: one forms a new layer of liber on the interior of all those which precede it, and the other a new ring of young sap-wood, exterior to all its predecessors. A portion of the cambium, in its unchanged or liquid state, always remains between the wood and the bark, which are never in absolute contact.

As a new cylinder of wood enclosing all its predecessors is annually formed, the section of a stem perpendicular to its axis exhibits a ring of woody fibre, alternating with a ring of spotted and rayed vascular tubes, which constitute the silver grain of the wood. The rings are more and more crowded, and narrower towards the centre, and at last become impervious to the sap, which only rises through the younger part of the sap wood. In fact, a large portion of the solid fibres of most plants have ceased to take any active share in the performance of vital functions, and, like the solid heart of an oak, retain their integrity simply because they are not exposed to influences which would cause their decomposition. A vegetable tissue exposed to ordinary chemical action, can only remain entire so long as it is performing vital functions. The arrangement of the woody fibre and ducts in the different orders and genera is much varied. The breadth of the rings of wood shows the effect of good and bad seasons; and in extra-tropical latitudes, where there is alternately a period of growth and repose, their number frequently indicates the number of years’ growth; so that the age of a tree may be approximately, if not exactly, learned from a critical examination of a section of its stem.

The innermost cylinder of wood is lined by the medullary canal or tube containing the pith. It is a delicate membrane, entirely formed of hollow spiral tubes. The pith, which fills the canal, is of greenish cellular tissue when young, full of sap, and occasionally, though rarely, mixed with vascular and spiral tissue. It passes uninterruptedly to the end of every branch, leaf bud, and flower. Perpendicular plates, called medullary rays, radiate from the medullary sheath and end at the bark, dividing the whole mass of the wood into triangular or wedge-shaped sections. They are thin plates of cellular tissue, stretched horizontally between the central pith and the bark. In each family of trees and shrubs they have a different arrangement, but in all they keep up a horizontal communication between the centre and the circumference, though they do not all extend throughout the whole length of the stem; some do, others do not. Thus the cellular tissue forms a horizontal system, while the fibro-vascular ducts constitute a perpendicular system of tissues. In some trees the pith is scarcely perceptible, and in others it diminishes or vanishes with age, as in the oak. In the alder and other plants it dries up, breaks into pieces, and the canal is filled with air.

In the stem and branches of the Coniferæ, there is scarcely any mixture of vessels amongst the woody fibre, the vascular system generally consisting exclusively of glandular woody tissue, except in the medullary sheath, where spiral vessels are found in small numbers.

The subterranean growth, or descending axis of trees consists of large branches, sometimes tending downwards, but more frequently spreading in extensive ramifications, not far from the surface of the earth. Their growth and structure are similar to those of the stem, but the cylinders of wood are less apparent; they have medullary rays, but no pith; they merely connect the active roots with the stem, and fix the plant firmly in the ground, for they have few or no pores, and contribute little to the nourishment of the plant, except by conveying liquids from the fibrous roots to the upper growth. The active feeding roots spring from them in the form of bunches of white fibres, like cords or threads, which sink straight down into the ground. These real roots are of cellular tissue enclosed in vascular tubes and spiral vessels, which terminate at a little distance from the extremity, leaving a point of loose spongy cellular tissue, called the spongiole, which absorbs from the ground the liquids that nourish the plant. These root fibrils are temporary organs; they die on the older parts of the subterranean branches, and are succeeded by others on the new.

The various tissues which form the stem of a tree form, in the same manner, though in diminished numbers, the complicated ramifications of the branches and the leaf-stalks, and terminate in the leaves themselves. Under the transparent film which forms the skin on the upper-surface of a leaf, there is a layer of soft thin-walled cylindrical or prismatic cells, closely pressed together, and full of green vegetable matter, or chlorophyll. Several layers of thick-walled cells follow, each more loosely aggregated than that which precedes it, and fuller of void spaces, till in the last green layer on the under-side of the leaf the cells are globular, with numerous large irregular void spaces, united in a reticulated system filled with air, and in direct communication with the atmosphere by means of the innumerable stomata, which are to be found in the under-surface of the leaves of all land plants of the higher classes, and which are their organs of respiration.

The form of the leaf is determined by the arrangement of the vascular bundles, which are in communication with those in the interior of the stem, and branch out in various directions through the green layers: these branches unite again, and form the skeleton of the leaf, which is often a delicate maze of the finest lacework of nerves. The vascular system is double, consisting of an ascending and descending portion. The ascending portion, which is continuous with the medullary sheath, becomes continuous at the apex of each nerve of the leaf with the descending portion, which is beneath and in contact with it throughout its ramifications. This descending portion at the base of the leaf-stalk, or petiole, becomes continuous with the bundles of the liber. In the upper part of the nerves of the leaf there are spotted vascular ducts, in the lower part there are laticiferous vessels. Those on the upper side carry the rising sap to the green matter, where it is elaborated and matured, and then it passes into the vessels on the under-side of the nerves or veins, which carry it down the liber.

Buds are generally formed of scales closely imbricated round the young leaves, which are variously folded and firmly packed; they contain the rudiments of the whole plant, and as in a large tree they are renewed every year, the sources of life are all but infinite.

The spines with which many plants are armed are of two kinds; one is permanent, being an excrescence from the wood, as in the blackthorn; the other proceeds from the bark, and may be stripped off, as in the rose; both contain silex, and are covered by the skin common to the whole plant.

Few plants of any kind are without hairs, which are chiefly found on the young shoots, and on the under surface of the leaves. They are either formed of a transparent elongated hollow cell, or consist of a number of transparent colourless superimposed cells, sometimes jointed, but more frequently rectilinear. When they sting, as in the nettle, they are set upon a kind of bulb composed of cells which secrete the acrid colourless liquid which causes the irritation, and when slightly pressed send it through the hair, the point of which breaks off as it enters the skin of the hand.