i6o MOSSES AND FERNS chap. 



short filament, which soon, at least when grown upon a solid 

 substratum, forms a flat thallus, which at first sometimes grows 

 by a definite apical cell.^ It first has a spatulate form (Fig. 

 y6, A, B), which later becomes broadly heart-shaped, and closely 

 resembles in this condition a young Fern prothallium, for which 

 it is readily mistaken. The older ones become more irregular 

 and may attain a diameter of several millimetres. The thallus 

 is but one cell thick throughout its whole extent, and is fastened 

 to the earth by colourless rhizoids. Later similar filaments 

 grow out from the marginal cells of the thallus, and a careful 

 examination shows that they are septate, and closely resemble 

 the protonemal filaments of other Mosses. Like those, the 

 septa, especially in the colourless ones, are strongly oblique. 

 These marginal protonemal threads may, according to Hof- 

 meister ^ and Schimper,^ produce a flattened thallus at their 

 extremity, and thus the number of flat thalli may be increased. 

 Schimper states that if the germination takes place in water, 

 the formation of a flat thallus is suppressed and the protonema 

 remains filamentous, but Goebel disputes this. 



In the few cases observed by me, only one leafy axis arose 

 from each thalloid protonema, and although this is not expressly 

 stated by Hofmeister and Schimper, their figures would indicate 

 it. At a point, usually near the base, a protuberance is 

 formed by the active division of the cells, in a manner probably 

 entirely similar to that in other Mosses, and this rapidly assumes 

 the form of the young stem. The first leaves are very simple 

 in structure, and are composed of perfectly uniform elongated 

 quadrilateral cells, all of which contain more or less chlorophyll. 

 Like the older ones, however, they show the characteristic two- 

 fifth divergence. Schimper states that the fifth leaf, at the latest, 

 shows the differentiation into chlorophyll-bearing and hyaline 

 cells, found in the perfect leaves. The first leaves in which 

 this appears only show it in the lower part, the cells of the 

 apex remaining uniform. 



At the base of the young plant very delicate colourless 

 rhizoids are developed, and these show the oblique septa so 

 general in the rhizoids of other Mosses. As the plant grows 

 older these almost completely disappear. 



The apex of the stem and branches is occupied by a 

 pyramidal apical cell with a very strongly convex outer free 



1 Goebel (12), p. ii. ^ Hofmeister (i). ^ Schimper (i). 



