Characteristics of the phloem and the cortex

Sclereids are present in all species. They occur in large groups in the prolongation of aggregate rays in Alnus, Carpinus and Corylus (Figs. 19-21), in round groups in Betula (Fig. 22) and in laterally elongated groups in Ostrya (Fig. 23). Sieve tubes and parenchy ma are arranged in indistinct tangential layers (Figs. 19 and 20). Sieve tubes are mostly collapsed in Alnus and Corylus. Prismatic crystals and crystal druses are frequent. The phellem consists of a belt of thick-walled, tangentially elongated cells (Fig. 24).

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Fig. 19. Phloem with indistinct tangential layers of collapsed sieve tubes and round parenchyma cells. The large sclerenchymat-ic complex lies in the extension of an aggregate ray. Stem of a 15 m-high tree, riparian, hill zone, Birmensdorf, Switzerland. Alnus incana, transverse section.

Fig. 20. Phloem with indistinct tangential layers of collapsed sieve tubes and round parenchyma cells. The large sclerenchymatic complex in the center lies in the extension of an aggregate ray. Stem of a 4 m-high shrub, hedge, hill zone, Birmensdorf, Switzerland. Corylus avellana, transverse section.

Fig. 21. Phloem, cortex and phellem. The large sclerenchymatic complex in the center lies in the extension of an aggregate ray. The cortex is characterized by sclerenchymatic bands containing prismatic crystals. Stem of a 10 m-high tree, Carpinus forest, hill zone, Zürich, Switzerland. Carpinus betu-lus, transverse section.

Fig. 21. Phloem, cortex and phellem. The large sclerenchymatic complex in the center lies in the extension of an aggregate ray. The cortex is characterized by sclerenchymatic bands containing prismatic crystals. Stem of a 10 m-high tree, Carpinus forest, hill zone, Zürich, Switzerland. Carpinus betu-lus, transverse section.

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Fig. 22. Phloem with indistinct tangential layers of collapsed sieve tubes and round parenchyma cells and with round groups of sclerenchyma cells. Stem of a 3 m-high tree, avalanche track, subalpine zone, Grisons, Switzerland. Betula pendula, transverse section.

Fig. 23. Tangentially elongated groups of sclerenchyma cells are embedded in a tissue consisting of sieve tubes and parenchyma cells. Stem of a 10 m-high tree, Carpinus forest, hill zone, Zürich, Switzerland. Carpinus betulus, transverse section.

Fig. 24. Cortex and phellem. The phellem (top) consists of thick-walled rectangular cork cells. Stem of a 4 m-high shrub, hedge, hill zone, Birmensdorf, Switzerland. Cory-lus avellana, transverse section.

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