22 HANDBOOK OF MOSSES. 



layer of cells of the spore sac. These two layers of cells, 

 when ripened, form those beautiful fringes which adorn the 

 mouths of many moss capsules, but in many other species 

 the peristome is absent or very rudimentary ; their presence 

 or absence, or whether single or double, are useful in the 

 discrimination of genera, and a study of their structure is in 

 some cases a valuable aid to the determination of species. 



By virtue of the insertion of the fruit-stalk, mosses are 

 divided into two sections, Acrocarpi, or those mosses which 

 have the fruit-stalk terminating the main stem (fig. 4), as 

 in Pottia truncata and Pleu-rocarpi, or those mosses which 

 have the fruit-stalk arising from the side of the stem (fig. 5), 

 as in Hypnum rutabulum. 



The fruit-stalk, which is always present, varies in length ; 

 in some cases, as in Phascum serratum, it is very short ; in 

 other cases it may be long and conspicuous ; it is usually 

 smooth, but sometimes the surface is distinctly roughened 

 or granulated, as in Hypnum rutabulum (fig 5, 2 b). It may 

 be straight or variously curved. 



The base of the fruit-stalk is surrounded by leaves, which 

 in some species differ remarkably in both form and structure 

 from the other leaves of the plant (fig. 5, 2 c). These are 

 the perichaetial leaves, and the character of these leaves 

 often forms a special feature in the description of mosses. 

 If these leaves are carefully removed, it will be seen that 

 the base of the fruit-stalk is surrounded by a membranous 

 sheath, the vaginula, already mentioned; this is usually 

 smooth, but in some species it is more or less clothed with 

 hair-like processes, and these minute differences are in some 

 cases great aids in the discrimination of nearly allied 

 mosses. 



At the top of the fruit-stalk is the capsule, or urn ; and 

 this organ presents great variety in its form, in some cases 

 globose, Phascum cuspidatum ; pear shaped, Leptobryum 

 pyriforme ; cylindrical, Tortula aloides ; straight, curved, or 

 erect, Tetraphis pellutida ; cernuous (curved to one side), as 

 in Hypnum rutabulum (fig. 5) ; or pendulous, as in many of 

 the Bryums; it may be smooth, striated, or furrowed. 



In some species the capsule is swollen all round at the 



