Xylem secondary

Lateral or secondary Fusiform, ray initials Produce secondary xylem and secondary phloem in woody plants... 

Figure I. Transverse section of secondary xylem of Persea borbonia — ...

Figure 2. Transverse section of coalified Persea secondary xylem for comparison with Figure I. At least three coalu products are shown dark colored cell inclusions, vessel wall derivatives, and pber-tracheid wall derivatives. 304X...

Bark is the multi-layered outer portion of the stems and roots of woody plants. While the word is used most often in referring to just the epidermis of a stem, bark actually includes all layers of the plant from the outside down to and including the vascular cambium. The vascular cambium is the only part of a stem that grows. Its undifferentiated cells divide rapidly, producing secondary phloem cells toward the outside of the plant and secondary xylem cells toward the inside (Figure 4.3). 

MENG, H., CAMPBELL, W.H., Substrate profiles and expression of caffeoyl coenzyme A and caffeic acid O-methyltransferases in secondary xylem of aspen during seasonal development, Plant Mol. Biol., 1998, 38, 513-520. 

In Gymnosperms the secondary xylem (wood) tissue of roots, stems and leaves consists either of punctated or scalariform cells, whereas in Angiosperms the secondary wood tissue may be varied in structural aspect. 

At about six weeks one. notes cells, dividing by tangential walls in the inner curve of phloem patches. This is in-trafascicular cambium. A single layer of flattened cells starts to cut off on its inner side a quantity of secondary xylem and pushes out the patches of bast fibers, adds a little secondary phloem on the outer side. Secondary... 

Patches of cells of the inner layer of pericamhium divide rapidly and are called, interfascicular cambium. These join the intrafasci-cular iambium to form a continuous cambium ring which then cuts off additional secondary xylem on its inner face and secondary phloem on its outer face pushing inward the first-formed or proto-xylem and outward the first-formed or protophloem. The medullary rays become deepened. 

Fig. 63.—Transverse section of California Privet root made about an inch and a half above the section shown in Pig. 6l and showing secondary structure. Note that epidermis, primary cortex and endodermis have completely disappeared. Cork (cfe) phellogen -ph) secondary cortex (rc) protophloem p ) secondary phloem cambium (c) secondary xylem ( ) and protoxylem ( ).

Pig. 64.—Photomicrograph of a transverse section of an old portion of California Privet root, showing completed secondary development. Note the prominent medullary rays (mr) cork (cfe) phellogen (,ph) secondary cortex (between ph and p ) protophloem (p )i secondary phloem ( ) cambium (c) secondary xylem tracheae (0 wood fibers (w/) and piotoxylem ( ). 

Between the bundles certain cells of the primary medullary rays become very active and form interfascicular cambium which joins the cambium of the first-formed bundles (intrafascicular cambium) to form a complete cambium ring. By the rapid multiplication of these cambial cells new (secondary) xylem is cut off internally and new (secondary) phloem externally, pushing inward the first-formed, or protoxylem, and outward the first-formed, or prolophloem, thus increasing the diameter of the stem. The primary medullary rays are deepened. Cambium may also give rise to secondary medullary rays. 

Secondary Phloem Soft Bast—phloem cells and sieve tubes. Cambium—active layer giving rise to secondary phloem on outer and secondary xylem or inner face, and adding to depth of med. rays. Secondary xylem—wood fibers, pitted vessels, tracheids. 

Fig. 71.—Portion of cross-section of four-year-old stem of Aristolochia sipho, as shown by the rings of growth in the wood. The letters are the same as in Pig. 68 but new tissues have been added by the activity of the cambium and a cork cambium has arisen from the outermost collenchyma cells and given rise to cork. The new tissues are I, cork cambium k, cork g, secondary phloem from the cambium, and just outside this is older crushed phloem , secondary xylem produced by the cambium m, secondary medullary ray made by the cambium (notice that this does not extend to the pith). Half of the pith is shown. Notice how it has been crushed almost out of existence. Compare Figs. 68 and 71, tissue for tissue, to find out what changes the primary tissues undergo with age, and to what extent new tissues are added. Photomicrograph x 20. (From Stevens.)...

Softwoods. The wood or secondary xylem of gymnosperms is composed of relatively few cell types see box). 

As a plant grows in diameter, secondary xylem is formed from the cambium. Auxin has been implicated in the control of both cambial division and the subsequent differentiation of tracheary element [47]. When vascular bundles are broken, parenchyma cells can redifferentiate into tracheary elements and restore the functional bundles this occurs in response to elevated auxin levels at the wound site [39]. 

There are situations where, during the course of normal cellular development, holes appear in the CMT array. One such example is during the formation of bordered pits in secondary xylem vessel elements [61]. Pit fields also develop in the maturing cell walls of the maize root cortex and may be pre-figured by the occasional MT holes that are normally seen in these cells. However, it is too early to say whether intracellular changes in gibberellin levels are related to these particular intracellular morphogenetic events which, in the case of secondary xylem vessels, would be rather localized to only a fraction of the cells within the secondary xylem tissue as a whole. 

When a BR biosynthesis inhibitor, brassinazole, was applied to Arabidopsis thaliana, high levels of ribulose-l,5-bisphosphate carboxylase-oxygenase proteins accumulated in the plastids of the cotyledons. These results suggest that brassinazole treatment in the dark induces the initial steps of plastid differentiation, which occur prior to the development of fhylakoid membranes. This is a novel presumed function of BRs [16]. Brassinazole treament also retards the development of secondary xylem in cress [17]. 

Sapwood Secondary xylem Sapwood that contains prosenchyma cells carries water and dissolved mineral cations from roots to inner cells antd it yields new wood every... 

In earlier work, [ H]-[9R]Z was supplied to the transpiration stream of both de-rooted and intact blue lupin (Lupinusangustifolius L.) plants [2]. Features of this work included (1) the very low proportion of [9R]Z which moved to the seed, although [9R]Z reached the pod walls in significant amounts and tended to be conserved therein (2) the direct lateral movement of [9R]Z and/or related cytokinins from xylem to bark (all tissues outside secondary xylem) (3) the detection of a metabolite, or closely related metabolites, of unknown nucleotide-like structure in pod walls, stem bark and developing lateral shoots. For convenience, the metabolites were all designated U-NT (meaning unknown nucleotides), because all... 

Hormones and Cuscuta Development Influence of Hormones on Secondary Xylem Differentiation, Phenylalanine Ammonia Lyase (PAL) Activity and Ligniflcation... 

Nature of Secondary Xylem in Free-Hanging Vines of Cuscuta... 

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