5 
Sphagnales. 
submerged plants. In the leaves borne on the main stem, which 
are soon covered up by the pendent branches and therefore become 
functionless, the green cells soon lose their chlorophyll, while the 
walls of the hyaline cells become perforated with holes of irregular 
size and outline. In the branch-leaves, the hyaline cells have their 
walls perforated to a smaller extent and in a more regular manner, 
the pores being circular or oval and having a thickened rim; these 
pores generally occur at the points where three cells meet, though 
they are also found along the sides of the hyaline cells where these 
border on the green cells. 
Fig. 58. Transverse section of growing-point of stem of Sphagnum 
acutifolium, showing in centre the apical cell and its youngest segments, and 
towards the periphery the differentiation of the leaf-cells. 
At first a section across a young leaf shows that the cells, 
arranged in a single layer, are all rectangular and of about tbe same 
size, but later the cross-section shows an alternation of large hyaline 
and smaller green cells (Figs. 58, 59). In some species the hyaline 
cells bulge towards the underside of the leaf, the green cells being 
then triangular in cross section with the base of the triangle 
directed upwards and about flush with the upper side of the leaf. 
In other species the relation between the two kinds of cells is just 
the reverse of this ; in others, again, the green cells are hemmed in 
