Occurrence and distribution of tracheoidal elements in the Orchidaceae

Eighty-eight species of West African orchids were examined for tratheoidal elements in the root, stem, pseudobulb and leaf. Forty-three species (49%) contained tiacheoidal elements, all were epiphytic species; none of the twelve terrestrial and one epilithic/terrestrial species examined contained tracheoidal elements. Of the epiphytic species, twenty-one contained tracheoidal elements in the roots; seventeen in the stem/pseudobulb and twenty-three in the leaves. The tracheoidal elements are similar in structure: they all possess annular, spiral, reticulate or pitted secondary wall thickenings. These elements vary in shape, size and distribution in the different species and these variations may be of some taxonomic use within the Orchidaceae. The function(s) of the tracheoidal elements is not clear, but they probably provide mechanical support and also aid water retention in the organs containing them.