Description
Plants of Buxbaumia have a much reduced gametophyte, bearing a sporophyte that is enormous by comparison. In most mosses, the gametophyte stage of the life cycle is both green and leafy, and is substantially larger than the spore-producing stage. Unlike these other mosses, the gametophyte of Buxbaumia is microscopic, colorless, stemless, and nearly leafless. It consists exclusively of thread-like protonemata for most of its existence, resembling a thin green-black felt on the surface where it grows. The plants are dioicous, with separate plants producing the male and female organs. Male plants develop only one microscopic leaf around each antheridium, and female plants produce just three or four tiny colorless leaves around each archegonium.
Because of its small size, the gametophyte stage is not generally noticed until the stalked sporangium develops, and is locatable principally because the sporangium grows upon and above the tiny gametophyte. The extremely reduced state of Buxbaumia plants raises the question of how it makes or obtains sufficient nutrition for survival. In contrast to most mosses, Buxbaumia does not produce abundant chlorophyll and is saprophytic. It is possible that some of its nutritional needs are met by fungi that grow within the plant.
The sporophyte at maturity is between 4 and 11 mm tall. The spore capsule is attached at the top of the stalk and is distinctive, being asymmetric in shape and oblique in attachment. As with most other Bryopsida, the opening through which the spores are released is surrounded by a double peristome (diplolepidious) formed from the cell walls of disintegrated cells. The exostome (outer row) consists of 16 short articulated "teeth". Unlike most other mosses, the endostome (inner row) does not divide into teeth, but rather is a continuous pleated membrane around the capsule opening. Only the genus Diphyscium has a similar peristome structure, although that genus has only 16 pleats in its endostome, in contrast to the 32 pleats in Buxbaumia. Diphyscium shares with Buxbaumia one other oddity of the sporophyte; the foot (stalk base) ramifies as a result of outgrowths, so much so that they may be mistaken for rhizoids.
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