Characteristics of the xylem

Annual rings are distinct in all species except Comandra umbel-lata. Ring distinctness is indicated by semi-ring porosity (Figs. 1-3). Earlywood vessel diameter varies from 30-60 pm and vessel density from 300-500/mm2. Simple perforations (Figs. 4 and 10) and fibers with distinctly bordered pits are characteristic of all species (Fig. 5). Parenchyma is apotracheal (Fig. 1) and often marginal in 5 Thesium species (Fig. 2).

Fibers are fairly thick-walled (Figs. 1-3), especially in Thesium arvense and T. bavarum. Ray width distinguishes species: rays are predominantly uniseriate in Thesium humile (Fig. 6), large (4-10 cells wide) in T. divaricatum and T. pyrenaicum (Fig. 9) and smaller (1-3 cells wide) in all other species (Figs. 7 and 8). All Thesium species have square or upright ray cells (Fig. 10). Rays are extremely heterocellular in Osyris alba. Lignification of ray cells is limited in most species (Figs. 2 and 6-9).

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Left Fig. 1. Six distinct annual rings in a semi-ring-porous xylem. Distinct ray dilatations and groups of sclereids are present in the bark. Stem of a 60 cm-high shrub, hedge, Mediterranean zone, Alps, France. Osyris alba, transverse section.

Right Fig. 2. Three distinct annual rings in a semi-ring-porous xylem with a large band of marginal, unlignified parenchyma. Ray dilatations and small groups of sclereids in the bark are characteristic. Root collar of a 10 cm-high herb, dry meadow, hill zone, eastern Alps, Austria. Thesium arvense, transverse section.

Left Fig. 3. Three distinct annual rings in a semi-ring-porous xylem. Ray dilatations occur in the bark. Root collar of a 10 cm-high herb, dry meadow, mountain zone, Alps, France. Thesium pyrenaicum, transverse section.

Right Fig. 4. Vessels with simple perforations and large pits with round or elliptic apertures (blue). Root collar of a 10 cm-high herb, dry meadow, hill zone, Alps, Austria. Thesium linophyllon, radial section.

Left Fig. 5. Fibers with large pits (>3 pm) and gash-like apertures. Stem of a 60 cm-high shrub, hedge, Mediterranean zone, Alps, France. Osyris alba, radial section.

Right Fig. 6. Uniseriate rays with extremely elongated cells. Root collar of a 12 cm-high herb, dry meadow, thermophile zone, subtropical climate, Gomera, Canary Islands. Thesium humile, tangential section.

Left Fig. 7. 1-4-seriate rays. Root collar of a 10 cm-high herb, dry meadow, mountain zone, pinyon, Rocky Mountains, Colorado, USA. Comandra umbellata, tangential section.

Right Fig. 8. 1-3-seriate rays. Stem of a 60 cm-high shrub, hedge, Mediterranean zone, Alps, France. Osyris alba, tangential f vr sc r sc r

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Left Fig. 9. Multiseriate, unlignified rays (5-10 cells wide) in semi-ring-porous wood with distinct annual rings. Root collar of a 25 cm-high herb, Quercus ilex forest, Mediterranean zone, Alps, France. Thesium lino-phyllon, transverse section.

Right Fig. 10. Rays with square and upright cells. And vessels with simple perforations. Root collar of a 12 cm-high herb, dry meadow, thermophile zone subtropical climate, Gomera, Canary Islands. Thesium humile, radial section.

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