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.