noun
definition
A vascular tissue in land plants primarily responsible for the distribution of sugars and nutrients manufactured in the shoot
Examples of phloem in a Sentence
The xylem and phloem also, rarely form perfectly continuous layers as they do in a solenostelic fern.
The phloem is generally Type.
Eventually the new phellogens reach the level of the secondary phloem, and are formed in the parenchyma of the latter, keeping pace in their inward march with the formation of fresh secondary phloem by the cambium.
The stem is monostelic, the arrangement of the xylem and phloem being collateral.
Finally, in the pinnae of the frond the centrifugal xylem may disappear, the protoxylem being now exarch in position and abutting on the phloem.
Phloem The plant tissue that transports soluble food and minerals around a plant body (if the plant has vascular tissue).
The sieve-tubes, with their accompanying parenchyma or stereom, constitute the tissue called phloem.
New tangential walls arise in the cells which are the seat of cambial activity, and an initial layer of cells is established which cuts off tissue mother-cells on the inside and outside, alternately contributing to the xylem and to the phloem.
When a given initial cell of the cambium has once begun to produce cells of this sort it continues the process, so that a radial plate of parenchyma cells is formed stretching in one plane through the xylem and phloem.
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 central cylinder of the root, in which there are several xylem and phloem strands, has around it a two-layered endodermis, the inner layer of which appears to take the place of a pericycle.
The single stele in the stem consisted of the phloem surrounding a solid central strand of xylem, the groups of protoxylem being situated at the projecting angles.
A meristematic zone forms a short distance outside the xylem, from which secondary tissue is developed both internally and externally; that to the inside contains both xylem and phloem elements.
The stem, from the ground tissue of which sclerenchyma is absent, has a complicated system of steles arranged in concentric circles; the thick roots, the central cylinders of which have several alternating groups of xylem and phloem, arise in relation to these.
Even the most delicate tissues, such as cambium and phloem, the endosperm of seeds, or the formative tissue of the growing-point, are frequently preserved cell for cell, both in calcareous and silicious material.
Transverse section of stem, showing triangular primary wood, secondary wood, remains of phloem, and primary cortex.
Transverse section of stem, showing the pith containing groups of sclerotic cells, the primary xylem-strands, secondary wood and phloem, pericycle and cortex.
U1 - U', leaf-traces, numbered according to the phyllotaxis, It 5 belonging to the lowest leaf of the five; ph, a group of primary phloem; pd, periderm, formed from pericycle.
In favourable cases remains of the cambium are found on the outer border of the wood, and phloem is also present in the normal position, though it does not seem to have attained any considerable thickness.
The structure is in all respects that typical of roots, as shown by the centripetal primary wood, and the alternation of xylem and phloem groups observable in exceptionally favourable young specimens.
The primary structure f ` is only found unaltered in the 1 1, 1 ` by means of a cambium set in very Il 111/1/1, 1 1 early, xylem being formed internally and phloem externally in a perfectly normal manner.
The stem is traversed by a single stele, with solid wood, without pith; the primary xylem is triangular in section, the spiral elements forming one or two groups at each angle, while the phloem occupied the bays, so that the structure resembles that of a triarch root.
Phloem, surrounding the wood, is recognizable in good specimens; in the cortex the main feature is the great development of periderm.
Phloem The plant tissue that transports soluble food and minerals around a plant body (if the plant has vascular tissue ).
The external conjunctive tissue is often arranged in relation to each bundle separately, the pericyclic fibres for instance, already referred to, being cften confined to the bands of pericyclic tissue abutting on the phloem of each bundle, while the Cortex and pith frequently form rays in the intervals between the adjacent bundles.
The camb-ium in the typical case, which is by far the most frequent, continues the primary differentiation of xylem and phloem in the desmogen strand (see above), or arises in the resting mesodesm or mesocycle and adds new (secondary) xylem and phloem to the primary tissues.
At celtain points the cambium does not give rise to xylem and phloem elements, but cuts off cells on both sides which elongate radially and divide by horizontal walls.
The injured cells die and turn brown; the living cells beneath grow out, and form cork, and under the released pressure bulge outwards and repeatedly divide, forming a mass Of succulent regenerative tissue known as callus, Living cells of the pith, phloem, cortex, &c., may also co-operate in this formation of regenerative tissue, and if the wound is a mere knife-cut in the bark, the protruding lips of callus formed at the edges of the wound soon meet, and the slit is healed overoccluded.
The xylem and phloem parenchyma consist of living cells, fundamentally similar in most respects to the medullary ray cells, which sometimes replace them altogether.
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 phtoem strand, or, as it is sometimes called, a phloem -island, in the midst of the xylem.
Strengthening tissue of all kinds (and sometimes even the phloem) is more or less rudimentary.
The phloem completely surrounds the xylem, which usually develops from two protroxylem groups.
In the roots of Ophioglossum and Botrychium and in the first formed roots of Helminthostachys an endophytic fungus is present, forming a mycorhiza - the stele in the larger roots has the usual radial arrangement of xylem and phloem; monarch roots occur in Ophioglossum.
The xylem and phloem are nearly always found in close association in strands of various shapes in all the three main organs of the sporophyteroot, stem and leafand form a connected tissue-system running through the whole body.
The type of siphonostele characteristic of many ferns, in which are found internal phloem, and an internal endodermis separating the vascular conjunctive from the pith is known as a solenostele.
When the pith is large celled, the xylems of the bundles are separated from it by a distinct layer of conjunctive tissue called the endocycle, and a similar layer, the pericycle, separates the phloem from the cortex.
The latter is often sclerized, especially opposite the phloem, and to a less extent opposite the xylem, as in the stem.
As a bundle is traced towards its blind termination in the mesophyll the peridesmic stereom first disappears, the sieve-tubes of the phloem are replaced by narrow elongated parenchyma cells, which soon die out, and the bundle ends with a strand of tracheids covered by the phloeotermic sheath.
Differentiation of the xylem progresses outwards, of the phloem inwards, but the two tissues never meet in the centre.
The protoxylems and the phloem strands are developed alternately, just within the outer limit of the young cylinder.
A considerable evolution in complexity can be traced in passing from the simplest forms of xylem and phloem found in the primary vascular tissues both among Pteridophytes and Phanerogams to these highly differentiated types.
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.
The water taken up by the root from the soil contains nitrogenous and mineral salts which combine with the first product of photo-synthesis - a carbohydrate - to form more complicated nitrogen-containing food substances of a proteid nature; these are then distributed by other elements of the vascular bundles (the phloem) through the leaf to the stem and so throughout the plant to wherever growth or development is going on.
This phenomenon follows injury to the phloem in the lower parts of the stem, preventing the downward flow of elaborated sap. The injury may be due to gnawing insects, and particularly to the fungus Corticium vagum, var.
Secondary xylem and phloem produced by a single cambium, or by successive cambial zones; no true vessels (except in the Gnetales) in the wood, and no companioncells in the phloem.
Resin-canals, which occur abundantly in the xylem, phloem or cortex, are not found in the wood of the yew.
The cylinder is surrounded by a mantle of one or more layers of parenchymatous cells, the pericycle, and the xylem is generally separated from the phloem in the stem by a similar layer, the mesocycle (corresponding with the amylom sheath in mosses).
The leaf-bundles are always collateral (the phloem being turned downwards and the xylem upwards), even in Ferns, where the petiolar strands are concentric, and they have the ordinary mesodesm and peridesm of the collateral bundle.
In each bundle, separating the xylem and phloem, is a layer of meristem or active formative tissue, known as cambium; by the formation of a layer of cambium between the bundles (interfascicular cambium) a complete ring is formed, and a regular periodical increase in thickness results from it by the development of xylem on the inside and phloem on the outside.
The soft phloem soon becomes crushed, but the hard wood persists, and forms the great bulk of the stem and branches of the woody perennial.