M0S6'ES. 



315 



(basal) part of this cell is however concerned in the formation of the outer layers 

 of tissue of the stem. The apical part of the papilla constitutes the apical cell of 

 the leaf; it forms two rows of segments by partitions perpendicular to the surface 

 of the leaf. The number of the segments thus formed, in other words, the 

 terminal growth of the leaf, is limited, and the formation of tissue from the 

 cells thus formed advances downwards, ceasing finally at the base. The whole 

 of the tissue of the leaf is sometimes (as in Fontinalis) a simple layer of cells ; 

 but very commonly a vein, i. e. a more or less broad bundle, is formed from the 

 base towards the apex, dividing the unilamellar lamina into right and left halves, 

 and consisting itself of several layers of cells. The vein is sometimes composed 

 of uniform elongated cells, but more often different forms of tissue become dif- 

 ferentiated in it,^ among which are often formed bundles of narrow thin-walled cells 

 similar to the central bundle of the stem, and these are sometimes continued to it 

 through the external tissue of the stem as foliar bundles (9^ Lorentz, /. c\). The 

 shape of the leaves of Mosses varies from almost circular through broadly lanceolate 

 forms to the acicular ; they are always sessile and broad at their insertion ; usually 

 densely crowded ; only on the stolons of some species, the pedicels of the cupules 

 of the gemmoe of Aulacomnion and Tetraphis, as well as at the base of some 

 leafy shoots, do they remain small and remote (cataphyllary leaves). In the neigh- 

 bourhood of the reproductive organs they usually form dense rosettes or buds, and 

 then not unfrequently assume special forms and colours. In Racopilum, Hypo- 

 pterygium, and Cyathophorum, there are two kinds of leaves, a row of larger 

 upon one side, and a row of smaller leaves upon the other side of the stem. The 

 leaves are not branched, but entire or toothed, rarely slit. In some kinds peculiar 

 outgrowths are formed upon the inner or upper surface of the leaves ; in Barhida 

 abides^ articulated capitate hairs. The lamina, which in other cases expands 

 right and left from the median plane, is, in Fissidens, expanded in the median 

 plane itself, proceeding from an almost sheathing base. The tissue of the leaf is, 

 with the exception of the central vein, usually homogeneous and composed of 

 cells containing chlorophyll, which sometimes project above the surface as mamillae ; 

 in the Sphagnacese and Leucobryum the tissue is differentiated into cells of definite 

 position, some containing air, others sap. 



The mode of branching of the stem of Mosses is apparently never dichotomous, 

 but also probably never axillary, although connected with the leaves. Even when 

 the branching is copious the number of lateral shoots is nevertheless usually much 

 smaller than that of the leaves ; in many cases the lateral branches are definitely 

 limited in their growth, leading sometimes to the formation of definite ramified 

 systems similar to pinnate leaves (Thuidium, Hylocomium). When the primary 

 shoot produces reproductive organs at the summit, a lateral shoot situated beneath 

 it not unfrequently displays a more vigorous growth, continuing the vegetative 

 system; and by such innovations sympodia are formed. It sometimes happens 

 that stolons, that is shoots either destitute of or furnished with very small leaves, 

 creep on or beneath the surface of the ground, elevating themselves at a later period 

 as erect leafy shoots. The mode of branching is very various, and is closely con- 

 nected with the mode of life. The morphological origin of the lateral shoots has 

 been carefully investigated by Leitgeb in the case of Fontinalis and Sphagnum, and 



