Differentiating xylem

Larson (4,5) fed 14C02 photosynthetically to Pinus resinosa, divided the differentiating xylem into several fractions, and counted the radioactivity of each cell wall component. From these studies, it was concluded that as tracheid maturation occurred, xylose deposition increased, whereas mannose remained relatively constant, and both arabinose and galactose decreased considerably. 

Figure 4. Distribution of silver grains in microautoradiograms of differentiating xylem of pine administered with precursors of lignin and hemicellu-lose.

Fig. 1-2. Transverse section of xylem and phloem of red spruce (P/cea rubens). CZ, cambial zone DP, differentiating phloem MP, mature phloem with sieve cells (sc) and tannin cells (tc) DX, differentiating xylem with ray cells and tracheids (tr) MX, mature xylem, earlywood (EW) with resin canals (rc), lined with epithelial cells (ec) LW, latewood. Note that each ray continuous from the xylem, through the cambial zone, and into the phloem. Light micrograph by L. W. Rees. Courtesy of Dr. T. E. Timell.

Eom TJ, Meshitsuka G, Ishizu A Nakano J (1987) Chemical characteristics of lignin in the differentiating xylem of a hardwood III Mokuzai Gakkaishi 33 716-723 Fengel D, Wegener G (1984) Wood chemistry, ultrastructure and reactions de Gruyter, Berlin,... 

Interference microscopy can be applied to the study of lignin distribution in wood, in differentiating xylem, and in chips during pulping. The only constraints are that the material can be embedded and sectioned, matched samples are available for calibration, and the specimen can be oriented to obtain transverse sections. 

Imai, T., and Terashima, N., 1992, Determination of the distribution and reaction of polysaccharides in wood cell-walls by the isotope tracer techniques. IV. Selective radio-labeling of xylan in magnolia Magnolia kobus) and visualization of its distribution in differentiating xylem by microautoradiography. Mokuzai Gakkaishi 38 693-699 (in English). 

Chen, R, Yasuda, S. and Fukushima, K. (1999) Evidence for a novel biosynthetic pathway that regulates the ratio of syringyl to guaiacyl residues in lignin in differentiating xylem of Magnolia kobus DC. Planta, 207, 597-603. 

Sato, T., Takabe, K. and Fujita, M. (2004) Immunolocalization of phenylalanine ammonia-lyase and cinnamate-4-hydroxylase in differentiating xylem of poplar. Comptes Rendus Biol, 327, 827-36. 

As the embryo matures, a procambial system differentiates throughout the hypocotyl and cotyledons. This system later gives rise to the components of the vascular system. This is foreshadowed by the development of differences in staining characteristics. Most commonly embryos have only differentiating xylem or phloem elements (Bisalputra and Esau 1964). 

We have already mentioned that wound tissue, a callus, first forms on the cut surface of explants from higher plants. Let us now consider an explant from the xylem. It also first forms a callus out of undifferentiated cells (Fig. 209). Then pockets of cambium develop in this callus. This cambium then differentiates, xylem elements being formed on the side towards the original xylem explant and phloem elements on the other side. Now let us do the same experiment with an explant from the phloem (Fig. 209). In this case the cambium forms phloem on the side oriented towards the phloem explant and xylem elements on the other. Evidently there exists a gradient of material from the explant to the callus which specifies and directs the differentiation of the vascular tissue. 

Thus the last word on homoeogenetic induction has still not been said. However, we can state with certainty that phytohormones not only play a role in stimulating division in an already existing cambium (pages 198, 202), they also participate in the differentiation of the cells generated by this cambium to form elements of the xylem and phloem. As usual, the interplay of several phytohormones among themselves and with other factors determines the kind of differentiation (xylem or phloem, etc.). 

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