Characteristics of the xylem

The characteristics of the two families are described together because no feature characterizes only one or the other family. The Apocyanaceae/Asclepiadaceae are anatomically a very heterogeneous taxonomic unit. 15 of the 21 species analyzed have distinct rings (Figs. 1-6). Most species are diffuse-porous (Figs. 1 and 2) and/or semi-ring-porous (Figs. 1, 3, 5). Only the Mediterranean liana Periploca graeca is ring-porous (Fig. 6). Vessel diameter normally ranges from 30 to 80 pm. It is less than 20 pm in the rhizome of Vinca minor (Fig. 1) and V major, and larger than 100 pm in the liana Periploca graeca (Fig. 6) and the desert shrub Calotropisprocera (Fig. 5). Vessel densities vary between <50/mm2 (Fig. 8) and >200/mm2 (Fig. 1).

Vessel cell walls are perforated by small alternating (Fig. 7) or rarely scalariform pits and simple perforations (Fig. 7). Most species have vestured pits. They seem to be absent in Vinca minor and V major, Vincetoxicum officinale and Caralluma quad-rangula. Fiber-wall thickness is extremely variable: very thin-r

Vincetoxicum Officinale

Fig. 2. Annual boundaries are marked by a row of radial flat fibers in the latewood. Xylem with semi-porous vessel distribution, apotracheal parenchyma and uniseriate rays. Shrub, canyon, succulent zone, subtropical climate, Gomera, Canary Islands. Nerium oleander, transverse section.

Fig. 1. Distinct annual boundary of a slightly semi-ring-porous xylem. Vessels are small (<20 pm) and numerous (>200/mm2). Rhizome of a perennial herb, garden, mountain subtropical climate, Gomera, Canary Islands. Vinca major, transverse section.

Fig. 2. Annual boundaries are marked by a row of radial flat fibers in the latewood. Xylem with semi-porous vessel distribution, apotracheal parenchyma and uniseriate rays. Shrub, canyon, succulent zone, subtropical climate, Gomera, Canary Islands. Nerium oleander, transverse section.

Fig. 3. Annual boundaries are marked by semi-ring-porous vessel distribution and thin-walled fibers in the earlywood. Root collar of succulent plant, succulent zone of the dry subtropical climate, Gomera, Canary Islands. Ceropegia fusca, transverse section.

Iwv ewv

Iwv ewv r

Diffuse Porous And Ring Porous Trees

Fig. 5. Annual boundary of a semi-ring-porous to ring-porous wood. Fiber cell walls are slightly thicker in the latewood than in the earlywood. Stem of a shrub, arid climate, Arabian desert, Oman. Calotropis procera, transverse section.

Fig. 4. Annual boundaries are marked by an unlignified marginal parenchyma. Stem of a succulent shrub, rock, tropical zone, Dhofar, Oman. Adenium obesum, transverse section, polarized light.

Fig. 5. Annual boundary of a semi-ring-porous to ring-porous wood. Fiber cell walls are slightly thicker in the latewood than in the earlywood. Stem of a shrub, arid climate, Arabian desert, Oman. Calotropis procera, transverse section.

Fig. 6. Distinct annual ring boundaries of a ring-porous xylem. Fiber cell walls are slightly thicker in the latewood than in the earlywood. Stem of a liana, thermophile zone, subtropical climate, Tenerife, Canary Islands. Periploca graeca, transverse section.

r walled in Adenium obesum (Fig. 8) and extremely thick-walled in Acokanthera oblongifolia (Fig. 9).

Axial parenchyma, as far as it could be observed, is apotracheal (Figs. 2, 8 and 9). The hemicryptophytes Vinca and Vincetoxi-cum (Fig. 10) are rayless. Ray width varies mostly from one- to 3-seriate rays (Figs. 11 and 12), but Leptadeniapyrotechnica has large multiseriate rays (Figs. 15 and 17). The xylem of most stems forms a closed circle. Only the succulent Caralluma quad-rangula remains in the form of a siphonostele (Fig. 13); the vas-f ivp p cular bundles are laterally isolated and the parenchymatic tissue in-between the vascular bundles can be interpreted as primary rays (Fig. 13). Ray cells are mostly upright or square. A special feature is the presence of sieve-cell groups (interxylary phloem) in Adenium obesum (Fig. 14), Leptadenia pyrotechnica (Fig. 15) and Rhazya stricta (Fig. 16). Interxylary phloem is diffuse in Leptadenia, diffuse and concentric in Rhazya and concentric in Adenium. In addition, Leptadenia has lacitifers in the rays (Fig. 17). Septate fibers are present only in Ceropegia fusca

Septate Fibers

v pa

Fig. 9. Solitary vessels in a very thick-walled fiber tissue. Parenchyma is apotracheal. Stem of a cultivated shrub, succulent zone, subtropical climate, Tenerife, Canary Islands. Acokanthera oblongifolia, transverse section.

v pa

Fig. 7. Small, round, alternating inter-vessel pits and a simple perforation. Stem of a cultivated shrub, succulent zone, subtropical climate, Tenerife, Canary Islands. Acokanthera oblongifolia, radial section.

Fig. 8. Few radial multiple vessels (< 50/ mm2) in a very thin-walled tissue. Many apotracheal parenchyma cells lie between fibers. Stem of a succulent shrub on a rock, dry, subtropical climate, Dhofar, Oman. Adenium obesum, transverse section.

Fig. 9. Solitary vessels in a very thick-walled fiber tissue. Parenchyma is apotracheal. Stem of a cultivated shrub, succulent zone, subtropical climate, Tenerife, Canary Islands. Acokanthera oblongifolia, transverse section.

Acokanthera

Fig. 10. Rayless xylem. Rhizome of a pe- Fig. 11. Uniseriate rays. Shrub in a can- Fig. 12. Uniseriate to 3-seriate rays. Stem of rennial herb, garden, mountain subtropical yon, succulent zone, subtropical climate, shrub, arid climate, Arabian desert, Oman.

climate, Gomera, Canary Islands. Vinca Gomera, Canary Islands. Nerium oleander, Calotropisprocera, tangential section.

major, tangential section. tangential section.

Fig. 10. Rayless xylem. Rhizome of a pe- Fig. 11. Uniseriate rays. Shrub in a can- Fig. 12. Uniseriate to 3-seriate rays. Stem of rennial herb, garden, mountain subtropical yon, succulent zone, subtropical climate, shrub, arid climate, Arabian desert, Oman.

climate, Gomera, Canary Islands. Vinca Gomera, Canary Islands. Nerium oleander, Calotropisprocera, tangential section.

major, tangential section. tangential section.

Succulent Transverse Section

Fig. 15. Sieve-tube groups in the xylem are surrounded by parenchymatic cells. Sieve tubes are collapsed in central parts of the stem. Stem of a shrub, hyperarid climate, Algerian desert, Sahara. Leptadenia pyrotechnica, transverse section.

Fig. 13. Single bicolateral vascular bundle in a siphonostele. The bundles are laterally unconnected. The centripetal cell groups represent medullary sieve tubes. Succulent plant growing on a rock, dry subtropical climate, Dhofar, Oman. Caralluma quadrangula, transverse section.

Fig. 14. Sieve-tube groups in the xylem. Stem of a succulent shrub on a rock, subtropical climate, Dhofar, Oman. Adenium obesum, transverse section.

Fig. 15. Sieve-tube groups in the xylem are surrounded by parenchymatic cells. Sieve tubes are collapsed in central parts of the stem. Stem of a shrub, hyperarid climate, Algerian desert, Sahara. Leptadenia pyrotechnica, transverse section.

sf pa csi

Fig. 13. Single bicolateral vascular bundle in a siphonostele. The bundles are laterally unconnected. The centripetal cell groups represent medullary sieve tubes. Succulent plant growing on a rock, dry subtropical climate, Dhofar, Oman. Caralluma quadrangula, transverse section.

shc sf pa csi f v

Xylem Lignified Walls Pits Microslide

250 Mm

Fig. 16. Sieve-tube groups in the xylem are surrounded by parenchymatic cells. Stem of a succulent shrub, subtropical climate, Dhofar, Oman. Adenium obesum, transverse section.

Fig. 17. Very large ray with laticifers. Stem of shrub, hyperarid climate, Algerian desert, Sahara. Leptadenia pyrotechnica, tangential section.

Fig. 18. Septate fibers. The horizontal walls in fibers with small, bordered pits are un-lignified (blue). Stem of a succulent shrub on a rock, tropical climate, Dhofar, Oman. Adenium obesum, radial section.

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Responses

  • doderic
    What is a radial adenium obesum?
    4 years ago
  • pearl
    What are the characteristics of xylem?
    3 years ago

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