152 



MUSCINE& 



the single layer of cells of which it is composed becomes differentiated 

 into cells of two distinct kinds. A comparatively small number, of a 



lozenge-shaped form, grow to a large 

 size and lose the whole of their con- 

 tents, while their walls are provided 

 with spiral thickening-bands, and fre- 

 quently display a large circular orifice 

 opening from one cell into the next. 

 A much larger number of cells remain 

 permanently of a very small size, are very 

 narrow in proportion to their length, 

 and, being filled with protoplasm and 

 chlorophyll, constitute the whole of the 

 nutritive tissue of the leaf. These 

 nutritive cells form a kind of network 

 ramifying among the large empty cells ; 

 but, as their total area is small com- 

 pared with these latter, the entire leaf 

 has, to the naked eye, a semi-trans- 

 parent very light yellow-green appear- 

 ance. The tissue of the stem consists 

 of cells of three distinct kinds. In 

 the centre is a cylinder of tfcin-walled 

 elongated colourless parenchymatous 



FIG. 125. Sphagnum acKtifoUitm Ehrh. . . , , . . 



A, megaspore; B, microspore ; C, proto- Cells ; thlS IS enveloped in a layer Ol 



dotted prosenchymatous cells, the walls 



FIG. 126. Flat prothallium ofS. acutifotium, with young leafy stems (x 120). (After Schimper.) 



of which are thicker and of a brown colour ; while outside all is an 

 epidermal layer of large thin-walled empty cells, sometimes with spiral 



