INTRODUCTION TO CRYPTOGAMIC BOTANY. 467 



In Sphagnum and Leucohryum, there are two sets of cells, the 

 one filled with chlorophyll and very narrow (Fig. 98, h), the 

 other colourless. The walls of these have large round perfo- 

 rations, which in Sphagmim occur on the external wall, as 

 well as the others, while in Leucohryum (Fig. 99, h) they are 

 confined to the inner walls. In Odohlepharum there are 

 similar apertures, and in Leucophanes I find them at the tvi'o 

 extremities of the cells. In several white mosses there is more 

 than one layer of cells. In Leucohryum, the narrow cells 

 are imbedded amongst the colourless cells, but in Sphagnum 

 they are interstitial, so as in a direct view completely to sur- 

 round them. The leaves have often one or more nerves con- 

 sisting of elongated tissue, which when single may reach 

 the tip of the leaves or not, and run out in the form of 

 bristles. The margin is often serrated and thickened. The sur- 

 face is usually even, but occasionally rough with points. In 

 Polytrichum the upper surface of the nerve is lamellose ; in 

 Campylopus, the lower surface ; while in Pottia the laminae 

 are confined to a portion only of the upper surface. The leaves 

 are variously arranged, more frequently, however, so as to form 

 cylindrical masses of greater or less length, than flattened 

 strap-shaped branches. They very rarely have anything like 

 stipules; but in Hypopterygium (Fig. 98, d) there is decidedly 

 the same formation as in Jungermannioi ; while in the equi- 

 tant leaves of Fissidens there is a close resemblance to the 

 folded keeled leaves of Gottschea. I am not aware that 

 stomates occur on the leaves, but true stomates are found on 

 the female fruit of most mosses.* Unreliable spirals occur in 

 the calyptra of Mnium hornum, according to Schimper. 



515. The species of mosses are extremely numerous, and 

 many of them very widely distributed, though there are a 

 great many purely tropical or sub- tropical forms and genera. 

 Species which are too delicate for the temperate regions of 

 the northern hemisphere, extend sometimes through the whole 

 of South America to the Southern Islands. At the same time 

 many are able to endure very low temperature without injury ; 



* Valentine in Linn. Trans., vol. 18, p. 239. 

 SO * 



