l66 SECOND GROUP. MUSCINEAE. 



produce a branch of the protonema, the other basiscopic from which a moss-bud 

 may be formed ; one or both of them often developes rhizoid-filaments. The processes 

 of division here mentioned recall those which take place at the growing point of the 

 moss-stem itself, though there is no fundamental similarity between them. 



The apical cell of the stem is a three-sided pyramid in every genus except 

 Fissidens, where it is two-sided and produces two straight rows of alternating seg- 

 ments ; but here too the subterranean shoots grow with a three-sided apical cell, and 

 it is not till they come under the influence of light that the segmentation changes and 

 the leaves are arranged in two rows. In some species of Fissidens the lateral shoots 

 also which are formed above ground grow at first with a three-sided apical cell, and 

 the segmentation subsequently passes into that of a two-sided cell. The leaves are in 

 two rows also in the sterile shoots of Schzstofega * which look like the leaf of a Fern ; 

 but the apical cell is three-sided and the arrangement of the leaves is therefore spiral, 

 and afterwards becomes two-sided by displacement. The apical cell then, except in 

 Fissidens, is a three-sided pyramid with a convex base (Fig. 122); each segment of 



FIG. 119. Growth of rhizoids from protonema of Mnium hornuni with leafy buds k ; u<, iv the rhizoids of an in- 

 verted patch, from which the protonema-filaments n n are shooting forth, magn.go times. 



this cell arches outwards and upwards, forming a broad papilla which is then sepa- 

 rated off by a periclinal wall, the ' leaf- wall ' of Leitgeb, and developes by further 

 divisions into a leaf, while the lower and inner part of the segment produces by further 

 divisions a portion of the inner tissue of the stem. As every segment forms a leaf, the 

 arrangement of the leaves depends on the position of the consecutive segments; 

 in Fissidens the leaves alternate in two straight rows ; in Fontinalis they are in three 

 straight rows with a divergence of one-third, for the segments themselves are so 

 arranged, each new primary wall that is formed being parallel to the last but three, 

 and both belonging to one segment ; on the other hand in Polytrichum, Sphagnum, 

 Andreaea and others each new primary wall appears in the anodic direction of the 

 leaf-spiral ; the primary walls of a segment are not parallel, for the segments them- 

 selves, without any torsion of the stem, arise not in three straight rows, but in three 

 lines that wind spirally one above the other round the stem, and the consecutive 



Leitgeb, Das Wachsthum von Schistostega (Mittheil. cles naturw. Vereins zu Gratz, 1874). 



