ORIGIN AND NATURE OF VESSELS IN MONOCOTYLEDONS - 5 - ARACEAE SUBFAMILY COLOCASIOIDEAE

Tracheary elements from macerations of roots and stems of one species each of five genera of Araceae subfamily Colocasioideae were studied b y means of SER I (scanning electron microscopy). All of the genera hav e vessel elements not merely in roots, as previously reported for the family as a whole, but also in stems. The vessel elements of stems in all genera other than Syngonium are less specialized than those of roo ts; stem vessel elements are tracheid-like and have porose pit membran e remnants in perforations. The perforations with pit membrane remnant s demonstrate probable early stages in evolution of vessels from trach eids in primary xylem of monocotyledons. The vessel elements with such incipient perforation plates lack differentiation in secondary wall t hickenings between perforation plate and lateral wall, and such vessel elements cannot be identified with any reliability by means of light microscopy. The discrepancy in specialization between root and stem ve ssel elements in genera other than Syngonium is ascribed to probable h igh conductive rates in roots where soil moisture fluctuates markedly, in contrast with the storage nature of stems, in which selective valu e for rapid conduction is less. Syngonium stem vessels are considered adapted for rapid conduction because the stems in that genus are scand ent. Correlation between vessel element morphology and ecology and hab it are supported. Although large porosities in vessel elements facilit ate conduction, smaller porosities may merely represent rudimentary pi t membrane lysis. (C) 1998 The Linnean Society of London.