Characteristics of the phloem and the cortex

Characteristic of the family are the regularly arranged rectangular cells in the phellem (Figs. 36, 37 and 42) The phloem anatomy of the Rosaceae is heterogeneous. A uniform structure is characteristic of most hemicryptophytes; sieve tubes and parenchyma cells cannot be distinguished on transverse sections and sclerenchyma cells are absent (Fig. 37). Tangential bands of sclereids characterize the genera Cowania, Crataegus, Cydonia,

Exochorda, Pyrus and Sorbus (Figs. 38-41). Radially oriented groups of sclerenchyma were found in a few Prunus species as well as in Holodiscus (Fig. 42). Ray dilatations occur in all life forms but are rather indistinct (Fig. 39). Prismatic crystals or crystal druses were found in all life forms and in all subfamilies. Particular for all Prunus species is the dense band of persistent phellem cells which compresses and deforms the phloem (Figs.

Fig. 38. Phloem with almost continuous bands of sclerenchyma. Stem of a 4 m-high tree, cultivated, dry slope, Tbilisi, Georgia. Pyrus orientalis, transverse section.

Fig. 36. Layers of strictly radially oriented square and rectangular cork cells of the phellem. Rhizome of a 5 cm-high hemic-ryptophyte, meadow, subalpine, Briançon, France. Alchemilla alpina, transverse section.

Fig. 37. Fairly uniform phloem. Sieve tubes and parenchyma cells cannot be distinguished. Rhizome of a 10 cm-high chamae-phyte, bog, Yamal Peninsula, Siberia, Russia. Rubus chamaemorus, transverse section.

Fig. 38. Phloem with almost continuous bands of sclerenchyma. Stem of a 4 m-high tree, cultivated, dry slope, Tbilisi, Georgia. Pyrus orientalis, transverse section.

sc csi sc csi

Left Fig. 39. Tangential bands of sieve-tubes and parenchyma situated outside of the cambium. Sclerenchymatisation (red) occurs in the forth year after phloem formation. Stem of a 1.5 m-high shrub, dry meadow, mountain zone, Grisons, Switzerland. Rosa elliptica, transverse section.

Right Fig. 40. Tangential bands of scleren-chyma are interrupted by rays. Stem of a 4 m-high tree, hedge, mountain zone, Rigi, Schwyz, Switzerland. Sorbus aria, transverse x section.

Left Fig. 41. Tangentially arranged groups of sclerenchyma cells. Sclerenchymatisation occurs also in rays. Stem of a 2.5 m-high shrub, hedge, mountain zone, Bern, Switzerland. Crataegus monogyna, transverse

Right Fig. 42. An unlignified primary phloem and radially oriented sclerenchy-matic strips meet a tangential band of scler-enchyma cells of the cortex. The phellem consists of uniformly oriented rectangular cork cells. Stem of a high shrub, coastal Picea sitchensis forest, Oregon, USA. Holodis-cus discolor, transverse section.

Left Fig. 43. A dense band of phellem embraces the soft phloem zone. Only the turgescent phloem cells resist the phel-lem pressure. Stem of a 2.5 m-high shrub, hedge on a dry slope, hill zone, Schaffhausen, Switzerland. Prunus spinosa, transverse section.

Right Fig. 44. Curved rays and compressed sieve-tubes (dark red) are a result of the phellem pressure. Stem of a 5 m-high tree, cultivated, hill zone, Zürich, Switzerland.

Prunus amygdalus, transverse section.

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