142 MOSSES AND LIVERWORTS 



were intended to strengthen the point where the 

 two cell-walls meet. These are known as trigones ; 

 illustrations of them are given at Plate X. figs. 39 

 and 41, in the latter of which they are unusually 

 prominent, the leaf-cells also being specially large. 

 These trigones are not invariably met with in all 

 the species, and consequently their presence or 

 absence in any particular specimen is a matter 

 which will often be very helpful as a means of 

 identification. Trigones are never found in the 

 cells of moss leaves, though occasionally there is a 

 slight thickening of the cell-walls at the angles 

 which strongly reminds one of them ; this may be 

 seen in the leaves of some of the Thyme Thread- 

 mosses (Mnium). 



