702 THE SUBDIVISIONS OF THE VEGETABLE KINGDOM. 



The Luminous Moss just mentioned is an exception; in it the leaves are arranged in 

 two rows (c/. fig. 399 ^). The leaves of Mosses are generally simple, and (unlike the 

 Jungermanniaceae) provided with midribs. In many of the Polytricheae, and in 

 Barbula aloides, &e. (cf. fig. 895^), the upper surface of the leaf bears longitudinal 

 ridges of thin- walled chlorophyll-containing cells, thus adding to its assimilating and 

 transpiring surface. The Moss-plant can propagate freely by means of brood-bodies 

 and gemmge. These sometimes take the form of modified leaves, sometimes of little 

 stalked bodies on the leaves; occasionally they are collected together into little recep- 

 tacles at the tips of the shoots, as in Tetraphis (cf. fig. 196, p. 23, where this and other 

 cases are illustrated). The antheridia and archegonia are collected into little recep- 

 tacles or "flowers" placed either at the tips of the shoots (in the acrocarpous Mosses, 

 c/. figs. 397^ and 398^), or laterally in the leaf -axils (in the pleurocarpous Mosses, c/. 

 fig. 397 ^^). Occasionally both antheridia and archegonia are present together in the 

 same "flower" (cf. fig. 397 ^°), but more frequently they are in separate receptacles 

 (cf. figs. 398 * and 398 ^). Mingled with them are sterile scales, the paraphyses. The 

 structural details of the sexual organs and the mode of fertilization in Mosses has 

 already been described (cf. pp. 64-66). After fertilization the egg-cell within the 

 archegonium divides and enlarges, and gradually fashions itself into the sporogonium, 

 the asexual generation of the Moss. For a time the archegonium stretches with the 

 growing embryo, but sooner or later it is ruptured (cf. fig. 398^), and its upper 

 portion raised aloft on the sporogonium as the calyptra. Sometimes the calyptra 

 forms a closely-fitting cap, entirely investing the capsule as in Polytrichum (fig. 



397 ^), or it may be a little hood split down one side as in Bryum (figs. 397^ and 



398 ^). After the raising of the calyptra by the elongation of the stalk or seta of 

 the sporogonium the apex swells and develops into the capsule. Though in almost 

 all cases the sporogonium consists of a capsule borne on a long smooth stalk (the 

 seta), which is embedded below in the tissues of the female shoot of the Moss-plant, 

 a very considerable amount of variety is met with in the structural details of the 

 capsule itself. The seta may pass gradually into the capsule as in Bryum (fig. 

 397^), or there may be a bulb-like enlargement (apophysis) at the base of the 

 capsule as in Polytrichum (fig. 397^), or this enlargement may attain considerable 

 dimensions, exceeding the spore-producing part of the capsule, as in Splacknum 

 (fig. 399). This apophysis is of importance as an assimilating and transpiring organ, 

 and it is the only portion of the whole Moss which bears stomata. Within the 

 capsule is the spore-layer. This has the form of a hollow cylinder surrounding a 

 central sterile tissue, the columella. External to the spore-layer, and between it and 

 the wall of the capsule, is a lacuna generally traversed by chlorophyll-containing 

 filaments of cells. Above the spore-layer the columella expands into a mass of 

 tissue, which forms the lid of the capsule (operculum, cf. figs. 397^ and 397"). At 

 the periphery of the lid, where it abuts upon the wall of capsule, a ring of cells 

 becomes marked out (the annulus); later, by the rupture of this ring the lid comes 

 away, and the mouth of the capsule is guarded only by a set of teeth, the peristome 

 (cf figs. 397 ^ 397 ^ 399 3, and 399 s). By the time that the lid is ready to come 



