Primary phloem

The objectives of this paper are broad. Our first objective is to describe the primary mechanism of action of usnic acid on plants as ascertained by our laboratory.33 A second objective is to describe the phytotoxic activity of selected lichen anthraquinone analogues. In addition to the phytotoxic activity, we describe the effects of these secondary metabolites on phloem-feeding insects. Finally, we provide a hypothesis to explain the functional roles of these metabolites in the ecosystem. 

Jerusalem artichokes temporarily store assimilates in several locations within the plant that are in excess to the amount needed for structural and maintenance purposes. Most of these reserves are reallocated to the tubers during bulking. While a cross section of assimilates is found in these sites, carbohydrates predominate, of which inulin is the primary storage form. In addition to mono- and disaccharides and small amounts of starch, a number of nutrients are found, many of which are phloem mobile and reallocated to the tubers during the latter part of the growing season. 

Bark can roughly be divided into living inner bark or phloem and dead outer bark or rhytidome. The tissues of the bark substance are formed either by primary or secondary growth. The primary growth means direct production of embryonal cells at the growing points of the stem apex and their further development to primary tissues. Epidermis, cortex, and primary phloem are primary tissues (Fig. 6-1). The formation of secondary tissues... 

Buriat, V., Oudin, A., Courtois, M., Rideau, M. and St. Pierre, B. (2004) Co-expression of three MEP pathway genes and geraniol-lO-hydroxylase in internal phloem parenchyma of Catharanthus roseus implicates multicellular location of intermediates during the biosynthesis of monoterpene indole alkaloids and isoprenoid-derived primary metabolites. Plant., 38,131 1. 

Plants are predominantly composed of parenchyma and woody tissues. Parenchyma cells dominate the green tissues in leaves and are composed of a protein-rich protoplast surrounded by a cellulose wall. Woody plant cells dominate all support (sclerenchyma) and transport (xylem and phloem) stmctures in a plant. They are composed of several layers (middle lamella, primary wall, secondary wall, and tertiary wall) with varying proportions of cellulose, hemicellulose, and lignin (Fengel and Wegener, 1984). 

Venation is the pattern of veins in the blade of a leaf. The veins consist of vascular tissues which are important for the transport of food and water. Leaf veins connect the blade to the petiole, and lead from the petiole to the stem. The two primary vascular tissues in leaf veins are xylem, which is important for transport of water and soluble ions into the leaf, and phloem, which is important for transport of carbohydrates (made by photosynthesis) from the leaf to the rest of the plant. 

These are bands of parenchyma cells which extend radially from the cortex to the pith (primary medullary rays) or from a part of the xylem to a part of the phloem (secondary medullary rays). In tangential-longitudinal sections they usually appear spindle shaped while in radial-longitudinal sections they are seen crossing the other elements. Their primary function is to supply the cambium and wood with elaborated sap formed in the leaves and conveyed away by the sieve tubes, and phloem parenchyma and to supply the cam-... 

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.)... 

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 ( ).

Fig. 68.—Photomicrograph of cross-section of stem of Aristolochia sipko, where cambial activity is just beginning, a, Epidermis b, coUenchyma c, thin-walled parenchyma of the cortex, the innermost cell layer of which is the starch sheath or endodermis d, sclerenchyma ring of the pericycle e, thin-walled parenchyma of the pericycle /, primary medullary ray g, phloem h, xylem interfascicular cambium medulla or pith. X 20. (From Stevens.)...

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. 

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.)...

Fig. 74.—Part o a transverse section of a twig of the linden, four years old. m. Pith ms, medullary sheath x, secondary wood Ph, phloem 2, 3, 4, annual rings c, cambium pa, dilated outer ends of medullary rays b, bast pr, primary cortex k, cork. (From Sayre after Vines.)...

Pig. 212.—Cross section through root-bark of Euonymus atropurpureus. Note the two broad dome-shaped phloem patches, one on either side of a wedge-shaped primary medullary ray. 

Cor tex.—That region in dicotyl and gymnosperm roots of primary growth and in roots and stems of monocotyledons between epidermis and endcdermis, in dicotyl and gymnosperm roots of secondary growth or in barks between cork cambium and phloem. 

Even in its first few months of growth, a seedling already has well-developed tissues that function in translocation and physical support. These early but critically important tissues are called primary xylem and primary phloem and are produced by derivatives of apical meristems they are longitudinally arranged into tissue zones called vascular bundles (J, 2). These structures are the only circulatory... 

During the division and enlargement phases of wood cell development, the cell wall is a thin, deformable, and extensible envelope of material referred to as the primary wall. Near the cessation of cell enlargement, however, a secondary wall may begin to be manufactured to the lumen side of the primary wall. Wood fibers, vessel elements, and certain other xylem or phloem elements that function in passive conduction and/or support normally develop a secondary wall (Figure 5). 

Nicotine is mainly located in the intercostal areas of the leaf (247, 289, 290). Chojecki (291), by microchemical technique, has arrived at the following depots of nicotine in the tobacco plant given in the order of decreasing concentration leaf epidermis, particularly at the base of the hairs, spongy mesophyll tissue of the leaves, primary cortex of roots, epidermis and parenchymatous tissue of the stem and palisade tissue of the leaves small amounts in the phloem and xylem of the stem, in the veins of the leaves, and in the medulla of the stem and traces in flowers and axial cylinders of the roots. 

Deposition of the matrix substances and formation of the microfibrils are accompanied by a sequence of related processes that lead to the development and differentiation of the cell wall this sequence includes expansion of the wall, changes in the composition of the polysaccharides, organization and orientation of the different layers, deposition of callose for formation of pores in phloem, lignification (see p. 299), and other processes. Considerable information has been obtained about the mechanism of some of these processes and the factors that affect them this information has been reviewed by leading molecular biologists, and will very briefly be mentioned here because of its relevance to cell-wall formation and to the constitution of cell-wall polysaccharides of interest to carbohydrate chemists. According to the new concepts, the transformations of the cell wall are effected, or are assisted, by the presence of a variety of enzymes, proteins, and, perhaps, even ribonucleic acid to the extent that primary... 

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. 

Primary attraction has been demonstrated in several species. The clearest example is that of the ambrosia beetle Trypodendron lineatum which is attracted to ethanol produced under anaerobic conditions in dying trees (Moeck, 1970). Using host material cut or damaged to expose phloem and xylem tissues, primary attraction has also been demonstrated in four other ambrosia beetles, two Scolytus, two Dendroctonus, Ips typographus and five other species (Moeck, 1981). 

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

---