Ecological trends in the xylem and the bark

There is insufficient material to detect any ecological trends.

pa pith

Fig. 15. The small phloem is surrounded by a large cortex. Some thin-walled parenchymatic cortex cells contain crystal druses. The cortex is covered by a thick cuticula. Stem of a 10 cm-high, hemi-parasitic dwarf shrub on Juniperus sp. mountain zone, arid climate, Arizona, USA. Phoradendron juniperinum, transverse section.

Fig. 14. The phloem consists of thin-walled cells with large nuclei. The phloem of the primary vascular bundles is in the prolongation of the radial vessel rows. Indistinct dilatations separate the bundles. Characteristic is the irregular structure in the cortex, consisting of groups of sclereids, round cortex cells and thin-walled callus cells. The cortex is covered by a thick cuticula. Stem of a 30 cm-high, hemi-parasitic dwarf shrub on Prosopis glan-dulosa, hill zone, arid climate, Arizona, USA. Phoradendron californicum, radial section.

pith

Fig. 15. The small phloem is surrounded by a large cortex. Some thin-walled parenchymatic cortex cells contain crystal druses. The cortex is covered by a thick cuticula. Stem of a 10 cm-high, hemi-parasitic dwarf shrub on Juniperus sp. mountain zone, arid climate, Arizona, USA. Phoradendron juniperinum, transverse section.

Fig. 16. Vessel enclosures in the cortex. The vessels are characterized by lignification and scalariform inter-vessel pits. Magnified part of Fig. 15.

100 Mm

100 Mm

Left Fig. 17. Distinct triangular groups of phloem are in the continuation of the vessel groups in the xylem. A few conspicuous fiber groups are in the thin-walled paren-chymatic cortex. Bark of a 30 cm-high, hemi-parasitic dwarf shrub on Malus sylvestris, hill zone, Zürich, Switzerland. Viscum album, transverse section.

Right Fig. 18. A phloem with collapsed sieve tubes is in the prolongation of the radial vessel/parenchyma strips of the xylem. The xylem ray formation remains behind that of the vessels and parenchyma (Keilwuchs). The cortex consists of large irregular cells and is covered by a thick cuticula. Stem of a 30 cm-high, hemi-parasitic dwarf shrub on Proustia cuneifolia, mountain zone, arid climate, Uspallata, Argentina. Loranthus aphyllus, radial section.

Left Fig. 19. Xylem with phloem enclosures (thin-walled cells). Characteristic of the xylem are the radial vessel groups and the sclerenchymatic tangential row in the tangential band of parenchymantic cells. Stem of a tree, savannah, Western Australia. Nuytsia floribunda, transverse section.

Right Fig. 20. Breaking zone of a twig. The axial xylem of the young and old shoot is separated by an unlignified meristematic zone and a groove around the shoot. The old shoots, as well as the young axial ends of the shoots, are covered with a thick cuticula. An already broken shoot is decayed and compartmentalized by a barrier zone. Phoradendron juniperinum, radial section.

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