Velamentous roots

Several families contain members equipped with velamentous roots; an example is the aroid genus Anthurium. But this character is particularly refined in xeric Orchidaceae (Barthlott and Capesius 1975), both drought endurers and avoiders (Fig. 3.19A-G); thus it serves the majority of the epiphyte flora. The specialized nonliving rhizodermis (velamen) forms an insulating but permeable mantle, 1-24 cells thick, around a living core comprised of an often chlorophyllous cortex (Fig. 2.16) and central conductive stele. Cell walls within the velamen are elaborately pitted and sculptured, and sometimes differentiated into several anatomically distinct zones. Pit membranes rupture throughout the system, allowing extensive infiltration by microbes (Fig. 4.14). An epivelamen (Fig. 3.19B,D), if present, is delicate and, except at the pneumathodes, may disintegrate with age (Fig. 2.10). Although somewhat variable in structural detail, velamina are not very expandable or compressible; saturated or dry, a root has about the same diameter.

Upon contact with fluids, the velamen becomes engorged instantaneously by capillary flow. Only the pneumathodes remain air-filled, repelling moisture by some as yet undetermined mechanism. Fluids in a saturated velamen lie against the outmost layer of the cortex - a uniseriate, suberized exo-dermis, impermeable to water except through its transfer cells (Figs. 3.9, 3.19G). After moisture moves through the cortical parenchyma, another tissue is encountered - a stoutish endodermis which also contains passage cells. From there, the distance to conductive tissue is no more than the dimension of a few stelar parenchyma cells. Velamentous aroid roots are organized along the same lines, but nothing is known about function.

A fair number of epiphytic and a few terrestrial orchids produce tilosomes (Figs. 3.8, 3.9) consisting of numerous lamellate or fibrillar wall protrusions from the velamen just above passage cells (Benzing, Ott, and Friedman 1982b; Pridgeon, Stern, and Benzing 1983). Those comprised of intermesh-ing branches may act as one-way valves, readily admitting fluids but blocking vapor loss when dry and compacted. Coarser tilosomes (Pridgeon et al. 1983) seem less likely to modulate moisture exchange in this fashion, but perhaps they impede passage of pathogens through what would otherwise be the most easily breached point along the exodermis.

endodermis

Vanda Roots

transfer cell

--epivelamen

velamen -

exodermis endodermis

transfer cell

--epivelamen exodermis

transfer cell velamen -

spirally thickened idioblasts

Figure 3.19. The velamentous orchid root in partial transverse section (T.S.): (A) Kingidium taeniale; (B) Phalaenopsis amabilis; (C) Vanda parishii; (D) Campylocentrum sellowii; (E) Harrisella porrecta; (F) Encyclia tampensis. (G) View of the entire root tissue series (a = aeration cells of the exodermis). Figures are not drawn to a common scale.

cortex jitjp^iÉ

cortex collapses, contact with soil broken

velamen embolized, hjo uptake from wet substratum tissue h^o drawn to dry substratum cortex remains intact

Figure 3.20. A schematic representation of the desiccating effect of dry media adjacent to roots without (upper) and with (lower) a velamen. Dehydration of the nonvelamentous root exceeds that of the velamentous organ.

velamen embolized, hjo uptake from wet substratum tissue h^o drawn to dry substratum cortex remains intact

Figure 3.20. A schematic representation of the desiccating effect of dry media adjacent to roots without (upper) and with (lower) a velamen. Dehydration of the nonvelamentous root exceeds that of the velamentous organ.

Embolized (empty) velamina provide protection against water loss to hydrophilic substrata (matric forces generated by dry bark greatly exceed that of the driest air by many megapascals; Fig. 3.20). Jordan and Nobel (1984) observed that two nonvelamentous desert perennials, Agave deserti and Ferrocactus acanthodes, broke away from dry earth, collapsing the cortex and killing the roots. Regrowth and restoration of contact between plant and rewetted earth was affordable because less than 15% of total plant biomass had been sacrificed. Velamentous species do not need this mechanism, nor could those epiphytes facing drought shortly after every rainstorm bear the cost (e.g., bark orchids with small shoot/root ratios).

Although most authorities agree that aerial function is promoted by a velamen (but see Dycus and Knudson 1957), moisture retention varies, depending on specific anatomical details and overall form. The root's surface-to-volume (S/ V) ratio appears to be one of the most important factors: Among 10 species, S/ V ratios predicted drying rate better than did velamen thickness, number of velamen cell layers, cortical tissue compaction, or exodermal cell wall development (Benzing et al. 1983). Quite surprising was the discovery that roots of shootless Polyradicion lindenii, whose exodermal aeration cells are eroded like those of Campylocentrum sellowii (Fig. 3.19D) and lead directly into sizable intercellular airspaces, dried more slowly over CaCl2 than did the other nine species. Roots of this orchid are quite stout, but they bear a thin velamen of just two cell layers, the outmost of which largely disintegrates at maturity. Exodermal U cells (Fig. 4.12) and inner velamen cell walls, however, are very robust and must be unusually vapor-tight. Sanford and Adanlawo (1973) noted that epiphytic West African orchids native to arid locations featured deeper velamina and thicker-walled exodermal barriers than did those on mesic sites. Velamen depth may be more closely tied to procurement problems than to desiccation resistance. Should rain be scanty over long intervals, or roots hang free or cling to non-absorbent bark in wetter sites, a large velamen dead space that prolongs contact with precipitation could be crucial for survival.

Existing information on aerial orchid roots poses many interesting questions. Why, for instance, is there so much variety? Perhaps different aspects of root structure can be modified to achieve equivalent performance. Studies of whole-body integration and consistency within plant groups would be useful. Are roots of all deciduous forms sacrificed at the end of a wet season? Are they so constructed as to minimize anoxia that could accompany long-term engorgement of velamina in deep wet humus? What functional tradeoffs exist between requirements for absorption or photosynthesis on the one hand and drought resistance on the other, and what limits are thereby set on the physical dimensions of a given orchid's root system? Finally, how do nonvelamentous epiphytes in droughty microsites avoid root desiccation without impairing future absorptive function? Some clues on this last question may already exist in the anatomical literature. For instance, Wilder (1986) noted a more persistent epidermis and greater sclerenchyma development in first-order roots of predominantly epiphytic compared to terrestrial Cyclanthaceae.

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