Phloem cells

Fromm and Spanswick [79] studied inhibiting effects of DNP on the excitability of willow by recording the resting potential of the phloem cells. In willow, 10 " M DNP rapidly depolarized the membrane potential by about 50 mV. 

Fromm and Spanswick [79] found that electrical stimulation of a plant is followed by ion shifts which are most striking in the phloem cells. While their content of potassium and chloride was diminished after stimulation, the amount of cytoplasmic calcium increased slightly (Table 1). These displacements lead to the conclusion that Ca + influx as well as and CP efflux are involved in the propagation of action potentials. The main difference between propagation of action potentials in animals and plants is that in an axon there is the K /Na transmembrane transport but in phloem cells the K /Ca channels are involved in this process [Fig. 22(b)]. 

The border between wood and bark is the cambium, which consists of only one layer of cells. This living cell layer produces xylem cells towards the inside of the stem and phloem cells towards the outside (Figure 1). In all the wood species studied, common characteristics of the cambium include a high content of pectins and the absence or low content of lignin (39, 40,... 

Two complex tissues, the xylem and phloem, provide the conducting network or "circulatory system" of plants. In the xylem or woody tissue, most of the cells are dead and the thick-walled tubes (tracheids) serve to transport water and dissolved minerals from the roots to the stems and leaves. The phloem cells provide the principal means of downward conduction of foods from the leaves. Phloem cells are joined end to end by sieve plates, so-called because they are perforated by numerous minute pores through which cytoplasm of adjoining sieve cells appears to be connected by strands 5-9 pm in diameter.154 Mature sieve cells have no nuclei, but each sieve cell is paired with a nucleated "companion" cell. 

Cloning. Asexual propagation (cloning) of plants ordinarily occurs by virture of the ability of embryonic meristematic tissue to differentiate into roots and shoots. If isolated phloem cells or other more differentiated cells are cultured, the result is often the formation of a callus, a dedifferentiated mass of cells somewhat reminiscent of embryonic cells. Under proper conditions, e.g., in a coconut milk culture and in the presence of the correct auxin-to-cytokinin ratio, some carrot root phloem cells revert to embyronic cells and develop into intact plants.99 This experiment provided proof that the differentiated carrot phloem cells... 

Yang, N.-S. Russell, D. (1990). Maize sucrose synthase-1 promoter directs phloem cell-specific expression of GUS-gene in transgenic tobacco plants. Proceedings of the National Academy of Sciences (USA) 87, 4144-8. 

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). 

Radial growth begins in the cambium which is composed of a single layer of thin-walled living cells (initials) filled with protoplasm (cf. Fig. 1-2). The cambial zone consists of several rows of cells, which all possess the ability to divide. On division the initial cell produces a new initial and a xylem mother cell, which in its turn gives rise to two daughter cells each of the latter is capable of further division. More cells are produced toward the xylem on the inside than toward the phloem on the outside phloem cells divide less frequently than xylem cells. For these reasons, trees always contain much more wood than bark. 

Radial fibro-vascular bimdle, consisting of two phloem patches of phloem cells and sieve tubes on either side of two radial Jtylem arms of icylem cells, spiral tracheae and scalariform tubes. 

Phloem is that part of a fibro-vascular bundle that contains sieve tubes, phloem cells, and often bast fibers. 

Phloem cells Sieve tubes Companion cells... 

A radial fibro-vascular bundle of many alternating xylem and phloem patches and hence polyarch. The phloem tissue consists of phloem cells and sieve tubes. The xylem is composed of xylem cells, tracheae and wood fibers. 

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. 

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