MUSCI 



139 



(6) a reserve-system, usually represented by the aquiferous tissue; (7) 

 a secreting system, developed typically in the sporange. No special 

 secretory or excretory organs have been detected in mosses. 



The leaves of mosses originate as broad papillose bulgings of a cell 

 of the stem which becomes cut off by a septum. They are always of 

 small size, sessile, and vary in shape from extremely narrow to broadly 

 lanceolate or almost orbicular. The tissue of the greater part of the 

 leaf usually consists of only a single layer of cells, 

 all of which contain chlorophyll, except in the 

 Sphagnaceae and in Leucobryum, where the cells are 

 of two different kinds, one large and empty, the other 

 very small and chlorophyllous, thus giving the leaf 

 a very light yellow-green colour. In most mosses 

 the marginal cells of the leaf, and a row extending 

 through the middle of the leaf from the base to 

 the apex, are much smaller, and are disposed in 

 several layers, though still thin-walled, thus consti- 

 tuting the rudiments of an epiderm and midrib. 

 The midrib may even extend beyond the apex of 

 the leaf as an awn or bristle. The leaves are usually 

 crowded, especially in the neighbourhood of the 

 sexual reproductive organs. Their phyllotaxis is 

 spiral, or more rarely distichous. The branch- 

 ing of the stem of mosses is apparently neither 

 dichotomous nor axillary ; the number of lateral 

 shoots is always much smaller than that of the leaves. 

 When the primary shoot produces a so-called 

 ' flower ' at its apex, a lateral shoot situated beneath 

 it not unfrequently displays a more vigorous growth 

 of a monopodial character, and is then termed an 

 innovation. Prolification, or the prolongation of a 

 shoot by the continued growth of the bud within and 

 above the male ' flower,' is a common phenomenon 

 in Polytrichum. Nearly leafless shoots or stolons 

 are sometimes formed beneath or on 'the surface of 

 the ground, arising finally as erect leafy stems. 



In most mosses large numbers of rhizoids are formed on the under 

 side of the stem, often clothing it completely with a reddish brown felt 

 (see fig. 102). They differ from the protoneme in their tendency to grow 

 downwards, and in not usually containing chlorophyll ; but there is no 

 sharp distinction between the two, each possessing the power of pro- 

 ducing branches indistinguishable from those of the other. 



o 

 nified)! 11 



