Structure and Development of the Secondary Phloem in Populus euramericana GUINIER

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  • ポプラの二次師部の構造
  • ポプラ ノ 2ジシブ ノ コウゾウ

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Development and differentiation of the secondary phloem and the morphological changes after maturation were studied in Populus euramericana GUINIER. Thin sections about 2 μ and 5 μ thick were taken from epoxy-embedded materials. The former sections were observed with Fujita's iodine staining method and the latter with safranin staining. 1) The secondary phloem developed in the alternate sequence from sieve tubes-parenchyma band (SP) to fibers band (F) (Photos 1, 2, 3, 4). Secondary phloem development usually began with SP differentiation and ended with SP differentiation. The number of fibers groups in a growth increment was variable, but there was a tendency that the first fibers group (F1) was wider and made a more continuous tangential band than later ones (F2, F3). Form these facts growth increments were distinguishable in the secondary phloem (Table 1). 2) In the dormant cambial zone, phloem cells which stopped further differentiation after an unequal cell division were observed (photo 5). It may be considered that these divided cells would differentiate into sieve elements and companion cells respectively, at the beggining of next spring. 3) Dense spherical nuclei were usually found in mature sieve elements, most frequently near the sieve plate, and they had different appearances from the nuclei in differentiating sieve elements (Photos 6, 7). In the dormant phloem the nuclei in mature sieve elments became swollen and dissolved (Photo 8). 4) Phloem rays changed their form considerably in different parts of the secondary phloem (Photos 7, 8, 9, 10). They dilated obviously in the outer part of the secondary phloem. Most of the sclereids originated from ray cells (Photos 10, 11). 5) After fiber primordia were separated from the cambium by the enlarging sieve tubes (Photo 2), they underwent intrusive elongation and matured within the growing season (Photo 3, 4). Phloem fibers groups were always surrounded by crystal-containing sclerified cells. 6) Phloem fiber walls were observed to be of two layers under the crossed nicols (Photo 10). The fibril angle of the inner layer measured by polarizing microscope was 30°to 40°, and that of the outer layer was 60°to 70° in the opposite direction to the inner layer.

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