Mm

Fig. 15. Phloem with tangential layers of large unlignified parenchyma cells, layers of small sieve tubes (blue) and sclerenchyma cells (red). Stem of a 2 m-high shrub, glaciar forefield, subalpine zone, Lotschental, Valais, Switzerland. Salix appendiculata, transverse section.

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Left Fig. 16. Uniform phloem with a few groups of sclerenchyma cells. The structure is typical for individuals on stressed sites. Stem of a 1.5 m-high shrub, riparian, tim-berline in the boreal zone, Quebec, Canada. Salix lanata, transverse section.

Right Fig. 17. Uniform phloem with hardly any sclerenchyma cells. The structure is typical for individuals on extremely stressed sites. Below ground stem ofa prostrate dwarf shrub, snowbed, arctic zone, Svalbard, Norway. Salixpolaris, transverse section.

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Left Fig. 18. Dilatation in the phloem of a fast-growing tree. Stem of a 10 m-high tree, riparian, hill zone, Zürich, Switzerland. Salix purpurea, transverse section.

Right Fig. 19. Prismatic crystals and crystal druses in the phloem. Crystals are located mainly around the sclerenchyma groups. Stem of a 10 m-high tree, riparian, hill zone, Birmensdorf, Zürich, Switzerland. Salix purpurea, transverse section.

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Fig. 22. Irregular layers of phellem. Groups of sclerenchyma occur in the cortex and in the phellem. Stem of a 6 m-high tree, dry rock field, hill zone, Hohtenn, Valais, Switzerland. Populus tremula, transverse section.

Fig. 20. One small layer of phellem outside of the cortex. Stem of a 2 m-high shrub, glacier forefield, subalpine zone, Lötschen-tal, Valais, Switzerland. Salix appendiculata, transverse section.

Fig. 21. Three layers of phellem located outside of the cortex. Stem of a 1.5 m-high shrub, riparian, boreal zone, Quebec, Canada. Salix lanata, transverse section.

Fig. 22. Irregular layers of phellem. Groups of sclerenchyma occur in the cortex and in the phellem. Stem of a 6 m-high tree, dry rock field, hill zone, Hohtenn, Valais, Switzerland. Populus tremula, transverse section.

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