Secondary phloem

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

Cork (or Suber). The outer tissues of the stems of the cork oak or the exterior layers of the bark beneath the epidermis. In young stems it consists of epidermis, cortical tissues periderm and in older stems of secondary phloem periderm. Cork is used in some expls mixts described below... 

Cloth made from the inner bark (secondary phloem) of various plants is found throughout many of the tropical regions of the world. Although the origins of its production are unknown, they have been chronicled as early as the 6th-century B.C. in China (5). The bark cloth of the Pacific, more commonly referred to by the Polynesian word "tapa", has been produced from the inner bark of various species of trees of the genera Broussonetia R. (paper-mulberry), Artocarous (breadfruit), and Ficus (fig species) (6). While examples of each of these types are well documented, the... 

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). 

Imagawa H (1981) Study on the seasonal development of the secondary phloem in Lanx leptolepis Res Bull Coll Exp For Hokkaido Univ 38 31-44 Imagawa H, Fukazawa K (1978) Influence of wood extractives on UV microscopy for lignin determination Mokuzai Gakkaishi 24 583-586... 

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... 

Pig. 60.—Cross-section of a young root of Phaseolus muHi-florus. A, pr, cortex m, pith X, stele or central cylinder—all tissue within the pericycle, inclusive g, primary xylem bundles b, primary phloem bundles. B, cross-section of older portion of root lettered as in A b, secondary phloem, k, cork. (Stevens, after Vines.)... 

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.)...

Jute and kenaf fibers develop in the phloem, or bast, region of the stem of the plants, and they appear as wedge-shaped bundles of cells intermingled with parenchyma cells and other soft tissues (Figure 7.1) in the transverse sections of the stem. In the growing part of the stem, a circumferential layer of primary fibers develops from the protophloem, but, as vertical growth ceases in the lower parts, secondary phloem fibers develop as a result of cambial activity. In mature plants, which reach a height of 2.5-3.5 m and a basal diameter of about 25 mm, the secondary fiber accounts for about 90% of the total fiber bundles. 

Similar sequential development is seen in secondary phloem fibers. At 63 days after planting, there is little thickening of the fiber wall and the fibers thicken gradually with maturity from 73 days to 112 days after planting. 

In Hevea brasiliensis, rabber is largely formed and stored in the bark, in rings of latex vessels interspersed with the sieve tubes of the secondary phloem of the trank, branches, or roots. Laticifers also are present in the leaves, flowers. 

Primary phloem Secondary phloem Food conducting tissue made of prosenchyma cells that carries the carbohydrates produced by photosynthesis from the leaves to the roots. 

Tippett J T, Shea S R, Hill T C, Shearer B L 1983 Development of lesions caused by Phytophthora cinnamomi in the secondary phloem of Eucalyptus marginata. Aust J Bot 31 197-210... 

---