Vegatative Architecture of Desiccation-tolerant Arborescent Monocotyledons

Stefan Porembski

doi:10.5642/aliso.20062201.10

Abstract

Within the monocotyledons the acquisition of the tree habit is enhanced by either primary growth of the axis or a distinctive mode of secondary growth. However, a few arborescent monocotyledons deviate from this pattern in developing trunks up to four meters high that resemble those of tree ferns, i.e., their woody-fibrous stems consist mainly of persistent leaf bases and adventitious roots. This type of arborescent monocotyledon occurs in both tropical and temperate regions and is found within Boryaceae (Borya), Cyperaceae (Afrotrilepis, Bulbostylis, Coleochloa, Microdracoides), and Velloziaceae (e.g., Vellozia, Xerophyta). They have developed in geographically widely separated regions with most of them occurring in the tropics and only Borya being a temperate zone outlier. These mostly miniature lily trees frequently occur in edaphically and climatologically extreme habitats (e.g., rock outcrops, white sand savannas). Characterized by a high degree of desiccation tolerance and a certain amount of fire-resistance, these plants are xerophytes that among vascular plants possess a unique combination of ecophysiological and morphoanatomical adaptations (e.g., poikilohydry, roots with velamen radicum). Moreover, most lily trees tend to form clonal populations of considerable age by means of stolons or by basal branching, which provides substantial advantages for the rapid and long lasting occupation of suitable sites. A summary is given of the major vegetative traits that obviously evolved independently within Boryaceae, Cyperaceae, and Velloziaceae.

Highlights

Due to the lack of a cambium in most monocotyledons the growth of their shoot systems is limited (Holttum 1955)
Velloziaceae contain more desiccation-tolerant species than any other family, with over 200 species in about eight genera that are distributed in South America, tropical Africa, Madagascar, and on the southern tip of the Arabian Peninsula (Kubitzki 1998)
It is conceivable that within both Velloziaceae and in the genus Borya, the acquisition of desiccation tolerance and a tree-like habit has occurred only once. This assumption is supported by the fact that desiccation-tolerant arborescent monocotyledons (DAM) are present in all genera of Velloziaceae and occur both in the Old and New World tropics

Summary

Introduction

Due to the lack of a cambium in most monocotyledons the growth of their shoot systems is limited (Holttum 1955). An alternative mode of attaining a treelike habit has been achieved by a few arborescent monocotyledons in developing trunks up to four meters high that resemble those of tree ferns, i.e., their "woodyfibrous" stems consist mainly of persistent leaf bases and adventitious roots (Fig. 1-4). Extensive vegetative propagation, in combination with the considerable age of individual DAM, enables the long-lasting occupancy of rock outcrop habitats independent of reproductive success.

Results

Conclusion