Evolution in the Canary Islands. I. Phylogenetic Relations in the Genus Echium (Boraginaceae) as Shown by Trichome Development

Abstract

Twenty-three species of Echium from the Canary Islands and Madeira were studied in order to relate growth form to evolutionary status and ecology. As a first step, phylogenetic relationships were approached from the point of view of similarity of trichomes on the leaf epidermis. Trichomes of three basic categories, subdivided into many types, occur in various combinations, as many as six different types to a single adult leaf. The trichome complement of each species was compared with all others, degrees of similarity were calculated, and a chart of relationships was drawn up (fig. 17). Ontogenetic studies of young leaves and of juvenile foliage show that the most highly differentiated trichome complements are found in a group of shrubs with appressed silvery pubescence and a group of shrubs with coarse, glabrescent leaves with pustular spines; a third group containing annuals, perennial herbs, hapaxanths, and shrubs possesses a more generalized and undifferentiated trichome complement. These results are v...