Anatomy and evolution in the Macaronesian Sonchus subgenus Dendrosonchus (Compositae: Lactuceae)

The members of the genus Sonchus subgenus Dendrosonchus are woody perennials possessing the pachycaul construction which has been proposed as ancestral to that of modern trees and herbs. The endemic flora of Macaronesia, particularly the Canary Islands, is generally considered to be a Tertiary relict, although an opposing view concerning woody plants of island floras has recently been expressed. The anatomy of Dendrosonchus is examined and trends of evolution can be interpreted from the vessel elements of the secondary xylem. With the use of the length to breadth ratios of vessel elements a gradual trend from primitive to derived is illustrated for the members of each of the two sections of Dendrosonchus , which are considered to have evolved in parallel. The wood rays, features of the cypsela and stem anatomy are also shown to be useful in elucidating the evolution of a small, closely related group of plants. Sonchus arboreus in the section Atalanthus and S. brachylobus in the section Dendrosonchus are similar to the two ancestral types proposed for the subgenus. The most derived species are considered to be S. pinnatus subsp. palmensis in the section Atalanthus and S. acaulis in the section Dendrosonchus. The ancestral type for the genus is considered to have been woody and probably of the Dendrosonchus type.