142 HOFMEISTER, ON 



fills the larger square cells, disappears, after having become 

 turbid and gramous, but without dividing into bodies of 

 any definite form. The contents of these latter cells 

 become clear like water. A considerable expansion of the 

 leaf-cells now ensues, especially in the longitudinal direc- 

 tion. It begins at the apex of the leaf, and proceeds from 

 thence rapidly downwards. The cells of the margin, which 

 divided once, and only partially, by means of septa at right 

 angles to the edge of the leaf, cannot keep pace with the 

 increase in size of the numerous median cells ; the leaf 

 assumes more and more the form of a cap. At the same 

 time the first traces of the well-known annular and spiral 

 threads begin to be visible upon the inner walls of the 

 larger square cells. A longitudinal division of many (often 

 of all) of the cells with watery contents frequently precedes 

 the appearance of the threads,. especially in Sphagnum squar- 

 rosum, so that each two thread-cells lie near one another 

 (PI. XXIII, fig. 4). Not unfrequently, also, many of the 

 small chlorophyll-bearing-cells divide by transverse septa 

 (PL XXVIII, fig. 3). In the mean time the chlorophyll- 

 granules in the small cells, which form a complicated net- 

 work between the thread- cells, increase considerably in 

 size. 



In the greater number of mosses, e.g. Phascum, Bryum, 

 Hypnum, Polytrichum, whose early development has been 

 admirably figured by Nageli, the formation of the leaves 

 agrees in the principal feature that is to say, in the 

 nature of the repeated division of the one apical cell with 

 Sphagnum. An essential difference exists, however, in the 

 fact that the number of the cells in the leaves of these 

 mosses increases considerably by the repeated bisection of 

 the cells of the lower part of the leaf, even after the division 

 of the apical cell has ceased, after the latter cell and its 

 neighbours have expanded considerably, after the contents 

 have become transparent, and the walls of the cells of the 

 apex of the leaf have become considerably thickened. In 

 Sphagnum this supplementary multiplication of the cells of 

 the base of the leaf can never be distinguished. Upon the 

 last division of the apical cell the lateral margin of the leaf 

 consists of a number of cells (normally from eighteen to 



