Characteristics of the xylem

The family is divided into the clades Saxifraga s. str. and Heuch-era (Heuchera, Mitella, Tiarella, Tolmeia). Bergenia crassifolia is isolated. Since the anatomical differences are very striking, we present the clades separately.

Clade Saxifraga

Many perennial Saxifraga species have distinct annuals rings (12 out of 19 analyzed species; Figs. 1 and 2). Ring boundaries are often indicated by very fragile primary walls between the early-wood and latewood cells. Ring shake is therefore frequent (Figs. 1-3). Most species are diffuse-porous and only rarely slightly semi-ring-porous (Fig. 2). The diameter of the small, isolated, round and fairly thick-walled vessels (1.5-2 pm) varies from 15-35 pm (Fig. 3). Vessel density varies from 200-500/mm2. Vessel-cell walls with pseudoscalariform pitting (reticulate thickening) and simple perforations are characteristic of all Saxifraga members (Fig. 4). Vessel thickenings are almost annular in the Crassulaceae family and rather pseudoscalariform in Saxifraga species. Fibers could not be detected in perennial species. It is impossible to distinguish them from parenchyma cells in the transverse section and difficult to detect them in the radial section. Fibers occur only in the annual above ground shoot of Saxifraga bulbifera and Saxifraga tridactylites and form a compact ring in the latewood (Fig. 6). Parenchyma is pervasive in all perennial species (Fig. 5). Rays and crystals are absent.

Left Fig. 1. More than 10 recognizable rings in a diffuse-porous, rayless xylem. Separated layers (ring shake) consist of the latewood of the previous year and the ear-lywood of the following year. Rhizome of a 5 cm-high herb, meadow, subalpine zone, Alps, Switzerland. Saxifraga oppositifolia, transverse section.

Right Fig. 2. Distinct rings in a diffuse-porous, rayless xylem. Latewood-ring boundaries do not correspond with the tangential layers; the weak zone lies just after the ear-lywood. A layer consists of the latewood of the previous year and the earlywood of the following year. Ring counting has to be based on the occurrence of earlywood vessels. Rhizome of a 5 cm-high herb, dry rock, alpine zone, Alps, Switzerland. Saxifraga caesia, transverse section.

Left Fig. 3. Thick-walled vessels are embedded in pervasive parenchyma. Tangential cracks occur in middle lamella between neighboring axial parenchyma cells. Rays are absent. Rhizome of a 5 cm-high herb, dry rock, alpine zone, Alps, Switzerland. Saxifraga oppositifolia, transverse section.

Right Fig. 4. Unlignified, pseudoscalari-form lateral vessel wall pitting. Rhizome of a 5 cm-high herb, dry rock, alpine zone, Alps, Switzerland. Saxifraga moschata, radial section.

pith

Left Fig. 5. Thick-walled, lignified vessels are embedded in a thin-walled, pervasive, unlignified parenchyma. Rays are absent. Rhizome of a 5 cm-high herb, dry rock, alpine zone, Alps, Switzerland. Saxifraga oppositifolia, transverse section.

Right Fig. 6. A belt of lignified fibers (red) surrounds the xylem of vascular bundles. Stem basis of a 15 cm-high annual plant, dry meadow, hill zone, Alps, Switzerland.

Saxifraga bulbifera, transverse section.

Clade Heuchera

There is not enough material to make a sound classification, but the analysed 5 species belonging to 4 genera (Heuchera, Mitella, Tiarella, Tolemia) are clearly distinct from the clade Saxifraga: Annual rings are marked by semi-ring porosity (Figs. 7 and 8). Vessels have scalariform perforations (5-15 bars; Fig. 9). Intervessel pits are small and round (Fig. 10) or scalariform (Fig. 11). Vessels are embedded in the fiber tissue (Fig. 8). Fiber pits are distinctly bordered. Parenchyma, when recognizable, is paratracheal. Large rays separate fiber/vessel compartments and large rays consist of square and upright cells (sometimes confluent to the fiber tissue). Rays are absent in the fiber/vessel groups (Fig. 8). Crystal druses occur in the pith.

Bergenia crassifolia and B. ciliata

Large rays occur between large vascular bundles (Fig. 12). The xylem is composed of vessels and pervasive parenchyma (Fig. 13). Vessels have simple and scalariform perforations and gash-like to scalariform intervessel pits. Scalariform intervessel pits and scalariform perforations can often not be differentiated. Crystal druses are frequent in the pith and the rays.

Fig. 7. Ring boundaries are indicated by semiring porosity in the radial strips of vessel/fiber zones between very large rays. Ring boundaries are absent in the parenchymatic ray zones. Rhizome of a 10 cm-high plant, dry rock, alpine zone, Rocky mountains, Colorado, USA. Heuchera hallii, transverse section.

Fig. 7. Ring boundaries are indicated by semiring porosity in the radial strips of vessel/fiber zones between very large rays. Ring boundaries are absent in the parenchymatic ray zones. Rhizome of a 10 cm-high plant, dry rock, alpine zone, Rocky mountains, Colorado, USA. Heuchera hallii, transverse section.

Fig. 8. Semi-ring-porous xylem. Large rays separate vessel/fiber zones. Rhizome of a 15 cm-high plant, garden, hill zone, Botanical Garden Zürich, Switzerland. Tiarella cor-difolia, transverse section.

ivp ivp

Fig. 9. Transition between scalariform in-tervessel pitting and scalariform perforation. Rhizome of a 15 cm-high plant, garden, hill zone, Botanical Garden Zürich, Switzerland. Mitella ovalis, radial section.

ivp ivp

Transverse Section Bergenia Ciliata

r vab

Left Fig. 10. Vessels with small round in-tervessel pits and scalariform perforations. Rhizome of a 15 cm-high plant, moist conifer forest, hill zone, Coeur d'Alene, Idaho, USA. Tiarella unifoliata, radial section.

Right Fig. 11. Scalariform intervessel pitting and scalariform perforation. Rhizome of a 25 cm-high plant, garden, hill zone, Botanical Garden of Zürich, Switzerland.

Heuchera americana, radial section.

Left Fig. 12. Single vascular bundles are laterally separated by large rays in the first ring. Many pith and ray cells contain crystal druses and most cells contain dark substances (tannins?). Rhizome of a 25 cm-high succulent plant, Botanical Garden Halle, Germany. Bergenia ciliata, transverse section.

m Right Fig. 13. Fiber-less xylem of a vascular S bundle. Fairly thick-walled vessels are embedded in pervasive parenchyma. Rhizome of a 15 cm-high succulent plant, Botanical Garden Halle, Germany. Bergenia ciliata, transverse section.

r vab

Was this article helpful?

0 0
Building Your Own Greenhouse

Building Your Own Greenhouse

You Might Just End Up Spending More Time In Planning Your Greenhouse Than Your Home Don’t Blame Us If Your Wife Gets Mad. Don't Be A Conventional Greenhouse Dreamer! Come Out Of The Mould, Build Your Own And Let Your Greenhouse Give A Better Yield Than Any Other In Town! Discover How You Can Start Your Own Greenhouse With Healthier Plants… Anytime Of The Year!

Get My Free Ebook


Post a comment