VI. 



THE BRYALES 



191 



appendages of the stem. The inner cehs of the segments by 

 repeated longitudinal and transverse divisions form all the tis- 

 sues of the axis. The second division wall in the segment, like 

 that in Sphagnum, is at right angles to the first, but in Ambly- 

 stegium it extends the whole breadth of the segment. By this 

 division the outer of the two primary cells of the segment is 

 divided into an upper cell, from which the leaf develops, and a 

 lower one from which the outer part of the stem and the buds 

 are formed. The leaves grow from a two-sided apical cell 



Fig. 98. — Amhlystegium riparium, var. fluitatts. A, Median longitudinal section of a 

 strong shoot; x, apical cell; x', initial of a lateral branch, X250; B, transverse 

 section through the apex, X250; C, similar section through a young branch, X500. 



(Fig. 99), as indeed they seem-to do in all Mosses, and the 

 divisions proceed with great rapidity and the young leaves 

 quickly grow beyond and surround the growing point. In 

 Amhlystegium, as in all the typical Bryales, the leaf has a well- 

 developed midrib. The formation of this begins while the leaf 

 is very young and proceeds from the base. In the middle row 

 of cells (Fig. 99, C), a wall first arises parallel to the surface 

 of the leaf, and this is followed by a wall in the cell on the lower 

 side of the leaf (Fig. 99, D). By further divisions in all the 



