BEITISH MOSSES. 45 



Vast tracts of land in this country and throughout 

 Northern Europe and America are covered with plants of 

 this group, and large tracts which are now fertile agricul- 

 tural land, where they have entirely ceased to grow, have 

 in former times been occupied by them. The bogs of 

 Ireland, which are mainly constituted of Turf Moss, were 

 computed in 1819 by the Bog Commissioners to occupy 

 2,830,000 acres. No Moss has probably ever, at least in 

 the present state of the globe, played so large a part as the 

 Sphagnum or Turf Moss. 



Structure. It is to the peculiar structure of the Peat 

 Moss that this great part on the theatre of the globe is to 

 be attributed. 



Leaves. In the young leaves the component cells are all 

 alike ; then by a differential growth we are presented with 

 large cells (sometimes of a square or rectangular shape) 

 surrounded by narrower cells ; then chlorophyll forms in 

 these narrow cells, but is absent from the square cells ; 

 from these the contents disappear, and water or water-like 

 fluid occupies the whole cell ; subsequently annular and 

 spiral threads develop on the walls of the square cells. 

 The intimate structure of the leaf thus enables it to absorb 

 great quantities of water. 



But again, the shape of the leaves is in many species 

 adapted to the retention of water. By a retardation of the 

 lateral as compared with the mesial growth, the leaf 

 assumes a boat shape. Often the edges of the leaves are 

 turned over ; the leaf thus affords means of holding 

 water. 



Figs. 27 and 28 will enable the reader to follow 



