162 MOSSES. AND FERNS CHAP. 



a turf of growing Moss is turned upside down, the rhizoids 

 thus exposed, to the light very soon develop chlorophyll, and 

 grow out into normal protonemal filaments. 



In most of the Mosses the leaves show a one-layered lamina 

 traversed by a midrib, which may be quite small or very 

 massive. This midrib is made up in part of elongated thick- 

 walled sclerenchyma, and contains a conducting tissue. The 

 highest grade of development of the leaf is met with in the 

 Polytrichacece, where the midrib is very massive and peculiar 

 vertical laminae of chlorophyll-bearing cells grow out from the 

 surface of the leaf. In Buxbaumia the leaves are almost en- 

 tirely abortive. The peculiar leaves of Sphagnum will be re- 

 ferred to later, as well as the details of structure of the leaves 

 of other forms. 



The stem, except in the lowest forms, is traversed by a 

 well-defined central strand of conductive tissue, and in a few 

 of the highest ones, e. g. Polytrichum f there are in addition 

 smaller bundles, continuations of the midribs of the leaves, 

 recalling the "leaf -traces" found in the stems of Spermato- 

 phytes. 



The types of non-sexual reproduction among the Musci 

 are extraordinarily various, and a careful study of them shows 

 that the morphological connection between the protonema and 

 gametophore is a very intimate one, as they may arise recip- 

 rocally one from the other. With the exception of certain 

 resting buds developed from the protonema it appears (Goebel 

 (10), p. 170) that the formation of the leafy stem is always 

 preceded by the protonema. The latter arises primarily from 

 the germinating spores, but may develop secondarily from 

 almost any part of the gametophore or even in exceptional 

 cases from the cells of the sporophyte (Pringsheim (2) ; 

 Stahl ( i ) ) . From these protonemal filaments new gameto- 

 phores arise in the usual way. The gametophore itself, es- 

 pecially where it is large and long lived, by the separation of 

 its branches rapidly increases the number of new individuals. 

 This is especially marked in Sphagnum, where this is the 

 principal method of propagating the plants. Special organs 

 of propagation in the form of gemmse also occur, and these 

 may develop from the protonema or from the gametophore 

 Tetr aphis pellucida (Fig. 118) is a good example, showing 

 these specialised gemmae which after a time germinate by 



