Characteristics of the xylem

In the present material annual rings are absent. Peperomia cape-rata does not have secondary growth (Fig. 1). Therefore we characterize only the xylem of Piper methysticum and P. nigrum (Figs. 2 and 3). Vessels remain solitary (Fig. 2). Vessel diameter of Piper nigrum varies from 50-80 pm whereas it exceeds 100 pm in Piper methysticum. The vessel density is low (<100/ mm2; Fig. 4). Vessels contain exclusively simple perforations. Perforations stand more-or-less perpendicular to the stem axis. Inter-vessel pits are predominantly annular in the vascular bundles and scalariform in the secondary xylem (Fig. 5). Piper methysticum produces locally small, thin-walled, unlignified tyloses (Fig. 4). The radial walls of fibers are perforated by very small slit-like or round pits (<2 pm; Fig. 6). Fibers are in the majority of cases thin- or thin- to thick-walled. Fibers of Piper methysticum are storied. Septate fibers have been found in Piper nigrum (Fig. 7). Axial parenchyma is paratracheal (Fig. 4). It surrounds mostly the whole circumference of the vessels. Large rays with more than 10 cells separate the remaining vascular bundles (Figs. 2, 3, 8 and 9). Ray cells with small round pits are upright (Fig. 10). Medullary vascular bundles are found in all species and secretory canals exist in the pith of Piper methysticum and P. nigrum (Figs. 11 and 12).

500 Mm

Fig. 2. Xylem with radial vessel/fiber strips. The pith is delimitated from the xylem by a wavy band of thick-walled fibers. Stem of a 1 m-high, 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper methysticum, transverse section.

500 Mm

Fig. 1. Species without secondary growth. Characteristic are the isolated vascular bundles inside the cortex. Stem of a 100 cm-long, hanging, 2-4-year-old shoot, tropical greenhouse, Botanical Garden Basel, Switzerland. Peperomia caperata, transverse section.

f pa

Fig. 2. Xylem with radial vessel/fiber strips. The pith is delimitated from the xylem by a wavy band of thick-walled fibers. Stem of a 1 m-high, 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper methysticum, transverse section.

annular thickenings

500 Mm duct' med'ulfaiy'vaD Fig. 3. Open collateral vascular bundles are outside the pith; they are connected by an intervascular cambium. The pith is delimitated from the xylem by a wavy band of thick-walled fibers. Stem of an 80 cm-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper transverse section.

Fig. 1. Species without secondary growth. Characteristic are the isolated vascular bundles inside the cortex. Stem of a 100 cm-long, hanging, 2-4-year-old shoot, tropical greenhouse, Botanical Garden Basel, Switzerland. Peperomia caperata, transverse section.

f pa annular thickenings

Left Fig. 4. Large, solitary vessels are surrounded by paratracheal parenchyma. One vessel contains unlignified tyloses. There are very large rays between radial vessel/ fiber strips. Stem of a 1 m-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper methysticum, transverse section.

Right Fig. 5. Vessels with annular thickenings in a vascular bundle in the center of the stem. Stem of a 100 cm-long, hanging, 2-4-year-old shoot, tropical greenhouse, Botanical Garden Basel, Switzerland. Pep-eromia caperata, radial section.

Left Fig. 6. Vessels with scalariform pits and fibers with small pits with slit-like apertures. Stem of a 1 m-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper methysticum, radial section.

Right Fig. 7. Septate fibers with unligni-fied (blue) horizontal cell walls. Stem of an 80 cm-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper nigrum, radial section.

Left Fig. 8. High (>5 mm) large rays with >10 cells in width and with sheet cells. Cells are thin-walled. Stem of a 1 m-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper methysti-cum, tangential section.

Right Fig. 9. High (>5 mm) and large rays (>10 cells in width). Cells are thin-walled and partially unlignified (blue). Stem of an 80 cm-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland. Piper nigrum, tangential section.

Fig. 12. Medullary shizogneous secretory canal in the center of the pith. Stem of a 1 m-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland.

Piper methysticum, transverse section.

Fig. 10. Ray with thin-walled upright cells. Fig. 11. Pith with medullary vascular bun-

Stem of a 1 m-high 3-year-old shrub, tropi- dles and shizogneous secretory canals. Stem cal greenhouse, Botanical Garden Basel, of an 80 cm-high 3-year-old shrub, tropical

Switzerland. Piper methysticum, radial sec- greenhouse, Botanical Garden Basel, Swit-

tion. zerland. Piper nigrum, transverse section.

Fig. 12. Medullary shizogneous secretory canal in the center of the pith. Stem of a 1 m-high 3-year-old shrub, tropical greenhouse, Botanical Garden Basel, Switzerland.

Piper methysticum, transverse section.

Taxa characteristic features

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