2 F. Cavers. 
the structure of the main stem, and reaches the same height as the 
latter. 
Fig. 55. Portion of a tuft of Sphagnum aciitifolimi , nat. size. 
The stem, and each of its branches, grows by means of a three- 
sided apical cell (Fig- 57a), the segments becoming displaced very 
early from the three-rowed arrangement so that the leaves come to 
form usually five rows (Fig. 58). Each leaf grows for a time by a 
two-sided apical cell, from which new cells are cut off right and left 
in an extremely regular manner (Fig. 57c). In some cases, e.g., in 
some submerged forms, the cells of the leaf remain practically 
uniform in size and shape, all containing chloroplasts, but as a rule 
this condition is rapidly replaced by differentiation of the cells into 
two sets. Some remain narrow but grow in length, and are arranged 
so as to form a network, each mesh of which is occupied by a large 
cell. The narrow cells retain their original contents, the large ones 
lose their protoplasm and chloroplasts. This differentiation takes 
place in a remarkably regular fashion. Each cell of the young leaf 
divides so as eventually to cut off a cell which is directed to the tip 
of the leaf and another which faces either the right or left side of 
the leaf; this is shown somewhat diagrammatically in Fig. 57e. 
where the dotted line represents the central axis of the leaf. The 
cells facing the apex (A) and the sides (R, L) remain narrow, while 
the third cell (H) of the triad grows in all directions and forms one 
of the large hyaline cells (Fig. 57f). As each cell divides in this 
symmetrical manner we get a regular and beautiful network of 
