154 MOSSES AND FERNS chap. 



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 is obviously a conducting tissue. 

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

 Polytrichum, where the midrib is very massive and peculiar 

 vertical laminae of chlorophyll-bearing cells grow out from the 

 surface of the leaf. In Buxbauviia the leaves are almost entirely 

 abortive. The peculiar leaves of SpJiagnuni will be referred 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, there are in addition smaller 

 bundles, continuations of the midribs of the leaves, recalling 

 the " leaf-traces " found in the stems of Spermaphytes. 



The forms 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 ^ that the 

 formation of the leafy stem is always preceded by the pro- 

 tonema. 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." From these protonemal filaments new 

 gametophores arise in the usual way. The gametophore itself, 

 especially 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 SpJiagnum, 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. 

 TetrapJiis pellucida (Fig. 105) is a good example, showing these 

 specialised gemmee which after a time germinate by giving rise 

 to a protonema upon which, as usual, the gametophore arises 

 ^ Goebel (10), p. 170. ^ Pringsheim (2); Stahl (i). 



