Raylessness and paedomorphosis: losses and gains of xylem rays en route from procambium to vascular cambium

A recurring motif of Carlquist’s work is the identification of traits typical for primary xylem of angiosperms as a whole, but found in the secondary xylem of certain species. These traits together make up the ‘Carlquistian syndrome’. Convergent occurrence of the syndrome in plants with similar habits suggests its adaptive value. One of its components — raylessness — has received the most attention and has been regarded as the result of selection favouring stem rigidity. However, how raylessness (or the Carlquistian syndrome in general) arises ontogenetically has been little studied. Here, we report that in some plants secondary xylem resembles primary xylem of the same individual, and not that of angiosperms in general as observed by Carlquist. Based on literature and microslide surveys, we identified this prolongation of primary xylem patterns into secondary xylem in most major clades of non-monocot angiosperms, including magnoliids, asterids and rosids.