Branch primordia

An overview of current definitions of different stem structnres in pleurocarpous mosses is given, covering rhizoids, axillary hairs, branches, leaves, pseudoparaphyllia, and paraphyllia. ParaphyUia are usually considered as structures not concentrated around branch primordia, but our observations revealed that in Alsia, Cratoneuron, Leptodon, Leskea and Palustriella, as well as in the... 

However, rhizoids may also develop from scattered positions on the stem (e.g., in many Calliergonaceae), from around branch primordia, or from the stem just above the leaf axils, axillary rhizoids (e.g., in many Plagiotheciaceae). This means that they have their origin in cell I (Figure... 

FIGURE 13.4 The three most common types of arrangement of the first psendoparaphyllium around branch primordia in the Hypnales. In all three cases the stem apex is directed npwards. (A) The four-eleven-o clock type, which is the most common one, found in the Amblystegiaceae, Hylocomiaceae, Leskeaceae, etc. (B) The Brachytheciaceae and Meteoriaceae type, with the first psendoparaphyllinm pointing downwards, and the second and third at 120° to the first one. (C) The low spiral type known from taxa snch as Thamnobryum and Trachyloma. 

FIGURE 13.6 Pseudoparaphyllia between cathodic and anodic leaf comers (A)-(D), (G), and around branch primordia (E), (F), (H) in Leptodon smithii (Hedw.) F. Weber D. Mohr (CLC and ALC = cathodic and anodic leaf comers, respectively LD = leaf decurrency). 

Where branch primordia are found evenly distributed along the module, but development of primordia into secondary modules is localized or complanate, it can be supposed that the branching pattern seen reflects mechanisms involving control or release of dormancy. The role of auxins, specifically indolyle-3-acelic acid (lAA), in apical donfinance has been shown to be similar to that in vascular plants (see Cooke et al., 2002, for summary), and can be expected to have an important role in the suppression or release from dormancy of branch primordia. For example, if the apical cell of the primary module is producing auxins that suppress development, branch primordia will develop only when they are sufficiently far from the apical cell for the levels of auxin to have dropped to a certain level. Similarly, the density of secondary modules may also reflect auxin control of module development, with branch primordia remaining dormant where the density of secondary modules has reached a local optimum. There is also evidence that lAA can control development of body-plan (i.e., selective development of branch primordia) (Cooke et al., 2002). 

It is important to note that the main diagnostic characters of pleurocarps (gametangia position, sporophyte structure, presence of leaf-like structures around branch primordia and dormant buds, etc.) are usually not available in fossil collections. Thus, we have no other way than to be satisfied with the less taxonomically important characters, such as pattern of laminal cell areolation, density of foliage, presence/absence of costa and branching pattern. 

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