ANATOMY
OF
phloem do not form part of the phloem-parenchyma, but occupy the top and bottom cell-rows of the medullary rays, the middle rows consisting of ordinary starchy cells. The top and bottom rows of the xylem rays are often developed as irregularlythickened radially-elongated tracheids which serve for the radial conduction of water, and communicate with the ordinary tracheids of the secondary xylem by large bordered pits. The primary vascular tissues of Angiosperms are likewise nearly always simple, consisting merely of tracheae and sieve-tubes often associated with amylom. A characteristic peculiarity, both in the primary and secondary tissue, is that the proteid cells of the phloem are here always sister-cells of the leptoids and are known as companioncells. In the secondary tissues of Dicotyledons we may have, as already described, considerably more differentiation of the cells, all the varieties being referable, however, on the one hand to the tracheal or sieve-tube type, on the other to the parenchyma type. The main feature is the development of special vascular stereom and storage tissue. In some cases special secreting tissues, resin ducts, oil glands, laticiferous tissue, crystal sacs, &c., may be developed among the ordinary secondary vascular elements. The limit of each year’s increment of secondary wood, in those plants whose yearly activity is interrupted by a regular winter or dry season, is marked by a more or less distinct Annual line, which is produced by the sharp contrast between rings. the wood formed in the late summer of one year (characterized by the sparseness or small diameter of the tracheal elements, or by the preponderance of fibres, or by a combination of these characters, giving a denseness to the wood) and the loose spring wood of the next year, with its absence of fibres, or its numerous large tracheae. The abundance of waterconducting channels is in relation to the need for a large and rapid supply of water to the unfolding leaves in the spring and early summer. In Gymnosperms, where vessels and fibres are absent, the late summer wood is composed of radially narrow thick-walled tracheids, the wood of the succeeding spring being wide-celled and thin-walled, so that the limit of the year s growth is very well marked. The older wood of a large tree forming a cylinder in the centre of the trunk frequently undergoes marked changes in character. The living elements die, and the walls of all the cells often become hardened, owing to the deposit in them of special substances. Wood thus altered is known as heart-wood, or duramen, as distinguished from the young sap-wood, or alburnum, which, forming a cylinder next the cambium, remains alive and carries on the active functions of the xylem, particularly the conduction of water. The heart-wood ceases to be of any use to the tree except as a support, but owing to its dryness and hardness it alone is of much use for industrial purposes. The great hardness of teak is due to the silica deposited in the heart-wood, and the special colouring matters of various woods, such as yellow wood, ebony, &c., are confined to the heart-wood. In some cases the heart-wTood, instead of becoming specially hard, remains soft and easily rots, so that the trunk of the tree liequently becomes hollow, as is commonly the case in the willow. Heart-wood is first formed at very different epochs in the life of a tree, according to the species—e.g., after fifteen to twenty years in the oak, forty years in the ash, etc. 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 Cambium in separate bundles these are said to be closed. The in stems, 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/asciewZar. _ In that case either very little secondary tissue is formed, as m the gourds, some Ranunculacese, &c., or a considerable amount may be produced (clematis, barberry, ivy). In the latter event the ceils 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 menstem connecting the cambial layers of adjacent bundles, an interfascicular is formed in addition to the fascicular cambium. 1 he inteifascicular cambium may form nothing but parenchymatous tissue, producing merely continuations of the primary rays, ouch 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. This is the typical case m 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 broa (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.
PLANTS
417
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 Hanes, characteristic of tropical and subtropical 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 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 lake, or, as it is usually 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 tvrtsted 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 Qami)jum phloems, and forms new (secondary) phloem in ja roots 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 (6) 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 quite the phenogetl same fashion as, the cambium, and producing gnd like the latter an external and an internal second- peri(]erm. ary 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 ot closely - fitting layers of cells with completely suberized walls, S. I. — 53
