Ecological trends in the xylem

No correlations could be found between the distinctness of growth rings and the climatic and vegetation zones in which the plants grow. The few species with distinct rings grow in dry regions of the in subtropical climate (Suaeda fruticosa) and Mediterranean Salsola genistoides) regions. Fibres are absent only in herbaceous species of the humid subtropical climate and the temperate zone. No other ecological trends in any anatomical feature of the xylem or bark could be detected.

Fig. 27. Simple construction of the bark. Phloem and cortex consist of small, radially oriented, unlignified parenchyma cells. Some cells contain crystal druses (dark spots). Bark of the stem, 40 cm-high dwarf shrub, dry place, Mediterranean, Aragon, Spain. Krascheninnikovia ceratoides, transverse section.

Fig. 27. Simple construction of the bark. Phloem and cortex consist of small, radially oriented, unlignified parenchyma cells. Some cells contain crystal druses (dark spots). Bark of the stem, 40 cm-high dwarf shrub, dry place, Mediterranean, Aragon, Spain. Krascheninnikovia ceratoides, transverse section.

Fig. 28. Simple construction of the bark. Phloem and cortex consist of large, more-or-less radial orientated, succulent cells. The lignified tissue below the phloem is part of the xylem. Some cells contain crystal druses. 40 cm-high annual succulent plant, ruderal site, subtropical climate, Gomera, Canary Islands. Patellifolia patellaris, transverse section.

Fig. 29. The cambial zone consists of parenchyma and sieve tubes, the phloem itself only of parenchyma cells. Parenchyma cells of the cortex are enlarged succulent cells. Between phloem and cortex are a few ligni-fied sclerenchyma cells. 40 cm-high, annual, succulent plant, ruderal site, subtropical climate, Gomera, Canary Islands. Chenopo-dium murale, transverse section.

Fig. 28. Simple construction of the bark. Phloem and cortex consist of large, more-or-less radial orientated, succulent cells. The lignified tissue below the phloem is part of the xylem. Some cells contain crystal druses. 40 cm-high annual succulent plant, ruderal site, subtropical climate, Gomera, Canary Islands. Patellifolia patellaris, transverse section.

._cry metaxylem pith branch branch metaxylem pith

Left Fig. 30. Vascular bundles in the pith (medullary vascular bundles). Root collar of an annual plant, ruderal site, hill zone, Birmensdorf, Switzerland. Atriplex patula, transverse section.

Right Fig. 31. Pith without vascular bundles. Root collar of an annual, prostrate plant, windy coast, subtropical climate, Gran Canaria, Canary Islands. Cheneloides tomentosa, transverse section.

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