Page:EB1911 - Volume 21.djvu/773

Rh In many annual plants no cambium is formed at all, and the same is true of most perennial Pteridophytes and Monocotyledons.

When the vascular tissue of such plants is arranged in separate bundles these are said to be closed. The bundles of plants which form cambium are, on the contrary, called open. In stems with open bundles the formation of cambium and secondary tissue may be confined to these, when it is said to be entirely fascicular. In that case either very little secondary tissue is formed, as in the gourds, some Ranunculaceae, &c., or a considerable amount may be produced (clematis, barberry, ivy). In the latter event the cells of the primary rays are either merely stretched radially, or they divide to keep pace with the growth of the bundles. If this division occurs by means of a localized secondary meristem connecting the cambial layers of adjacent bundles, an interfascicular is formed in addition to the fascicular cambium. The interfascicular cambium may form nothing but parenchymatous tissue, producing merely continuations of the primary rays. Such rays are usually broader and more conspicuous than the secondary rays formed within the wedges of wood opposite the primary bundles, and are distinguished as principal rays from these narrower subordinate or fascicular rays (fig. 24). This is the typical case in most trees where the primary bundles are close together. Where the primary bundles are farther apart, so that the primary rays are wider, the interfascicular cambium may form several fairly broad (principal) secondary rays in continuation of certain radial bands of the primary ray, and between these, wedges of secondary xylem and phloem: or, finally, secondary xylem and phloem may be formed by the whole circumference of the cambium, fascicular and interfascicular alike, interrupted only by narrow secondary rays, which have no relation to the primary ones.

In a good many cases, sometimes in isolated genera or species, sometimes characteristic of whole families, so-called anomalous cambial layers are formed in the stem, either as an extension of, or in addition to, the original cambial cylinder. They are frequently associated with irregularities in the activity of the original cambium. Irregularity of cambium occurs in various families of woody dicotyledonous plants, mostly among the woody climbers, known as lianes, characteristic of tropical and sub-tropical forests. In the simplest cases the cambium produces xylem more freely along certain tracts of the circumference than along others, so that the stem loses its original cylindrical form and becomes elliptical or lobed in section. In others the secondary phloem is produced more abundantly in those places where the secondary xylem is deficient, so that the stem remains cylindrical in section, the phloem occupying the bays left in the xylem mass. Sometimes in such cases the cambium ceases to be active round these bays and joins across the outside of the bay, where it resumes its normal activity, thus isolating a phloem strand, or, as it is sometimes called, a phloem island, in the midst of the xylem. The significance of these phenomena, which present many minor modifications in different cases, is not fully understood, but one purpose of the formation of phloem promontories and islands seems to be the protection of the sieve-tubes from crushing by the often considerable peripheral pressure that is exercised on the

stems of these lianes. Sometimes the original cambial ring is broken into several arcs, each of which is completed into an independent circle, so that several independent secondary vascular cylinders are formed. The formation of additional cambial cylinders or bands occurs in the most various families of Dicotyledons and in some Gymnosperms. They may arise in the pericycle or endocycle of the stele, in the cortex of the stem, or in the parenchyma of the secondary xylem or phloem. The activity of the new cambium is often associated with the stoppage of the original one. Sometimes the activity of the successive cambiums simply results in the formation of concentric rings or arcs of secondary xylem and phloem. In other cases a most intricate arrangement of secondary tissue masses is produced, quite impossible to interpret unless all stages of their development have been followed. Sometimes in lianes the whole stem breaks up into separate woody strands, often twisted like the strands of a rope, and running into one another at intervals. An ordinary cambium is scarcely ever found in the Monocotyledons, but in certain woody forms a secondary meristem is formed outside the primary bundles, and gives rise externally to a little secondary cortex, and internally to a secondary parenchyma in which are developed numerous zones of additional bundles, usually of concentric structure, with phloem surrounded by xylem.

The cambium in the root, which is found generally in those plants which possess a cambium in the stem, always begins in the conjunctive

tissue internal to the primary phloems, and forms new (secondary) phloem in contact with the primary, and secondary xylem internally. In roots which thicken but slightly, whose cambium usually appears late, it is confined to these regions. If the development of secondary tissues is to proceed further, arcs of cambium are formed in the pericycle external to the primary xylems, and the two sets of cambial arcs join, forming a continuous, wavy line on transverse section, with bays opposite the primary phloems and promontories opposite the primary xylems. Owing to the resistance offered by the hard first-formed secondary xylem, the bays are pushed outwards as growth proceeds, and the wavy line becomes a circle. Opposite the primary xylems, the cambium either (a) forms parenchyma on both sides, making a broad, secondary (principal) ray, which interrupts the vascular ring and is divided at its inner extremity by the islet of primary xylem; or (b) forms secondary xylem and phloem in the ordinary way, completing the vascular ring. In either case, narrow, secondary rays are formed at intervals, just as in the stem. Thus the structure of an old thickened root approximates to that of an old thickened stem, and so far as the vascular tissue is concerned can often only be distinguished from the latter by the position and orientation of the primary xylems. The cambium of the primary root, together with the tissues which it forms, is always directly continuous with that of the primary stem, just in the same way as the tissues of the primary stele. The so-called anomalous cambiums in roots follow the same lines as those of the stem.

In nearly all plants which produce secondary vascular tissues by means of a cambium there is another layer of secondary meristem

arising externally to, but in uite the same fashion as, the cambium, and producing like the latter an external and an internal secondary tissue. This is the phellogen, and the whole of the tissue it gives rise to is known as periderm. The phellogen derives its name from the fact that its external product is the characteristic tissue known as cork. This consists typically of close-fitting layers of cells with completely suberized walls, intended to replace the epidermis as the external protective layer of the plant when the latter, incapable as it is of further growth after its original formation, is broken and cast off by the increase in thickness of the stem through the activity of the cambium. Cork is also formed similarly in the root after the latter has passed through its primary stage as an absorptive organ, and its structure is becoming assimilated to that of the stem. The internal tissue formed by the phellogen is known as phelloderm, and consists usually of ordinary parenchyma. The phellogen may arise, in the first place, in any tissue of the axis external to the actual vascular tissues—i.e. in the epidermis itself (rarely), in any layer of the cortex, or in the pericycle. Its most usual seat of origin in the stem is the external layer of the cortex immediately below the epidermis; in the root, the pericycle. All the tissues external to the cork are cast off by the plant. The extent of development of the phelloderm is dependent upon whether the phellogen has a superficial or a deep-seated origin. In the former case the formation of phelloderm is trivial in amount; in the latter, considerable, since this tissue has to replace the cast-off cortex, as a metabolic and particularly a storage tissue.

Provision is made for gaseous interchange between the internal tissues and the external air after the formation of cork, by the development

of lenticels. These are special organs which interrupt the continuity of the impermeable layer of ordinary cork-cells. A lenticel is formed by the phellogen at a given spot dividin very actively and giving rise to a loose tissue of rounded cells which soon lose their contents, and between which air can pass to the tissues below (fig. 25). A lenticel appears to the naked eye as a rounded or elongated scar, often forming a distinct prominence on the surface of the organ. The lenticels of the stem are usually formed beneath stomata, whose function they take up after the