Spirina, U.N., M. Shimamura & M.S. Ignatov. On the branch primordia structure in the basal pleurocarpous mosses

Arctoa (2012) 21: 221-236

The development of leaves on branch primordia is studied in Ptychomniales, Hookeriales, and basal families of the Hypnales, including the Trachylomataceae, Plagiotheciaceae, Acrocladiaceae, etc. Many of them are characterized by lacking pseudoparaphyllia. However, the definition of this character remains vague. In order to avoid misleading terminology, we suggest distinguishing, with certain refinements, the Bryum-type and Climacium-type of branch primordia. Their main difference concerns the origin of the most proximal branch leaves: in the Climacium-type, they are derived from cells that are the first merophytes produced by the branch apical cell, while in the Bryum-type, the first merophytes do not produce leaves and the first branch leaves appear on branch primordia from cells that are later descendants of the branch apical cell. The Bryum-type is often associated with a leaf deep splitting to its base into separate segments, and appearing as independent structures (and sometimes referred to pseudoparaphyllia) although originating from a single merophyte as a compound leaf. Bryum-type branch primordia are characteristic of basal groups of pleurocarpous mosses, while Climacium-type is represented in most of advanced families.