14 HANDBOOK OF MOSSES. 



may be nerveless, as in Hypnum stellatum. The nerve is 

 of variable length, in some cases vanishing below the tip of 

 the leaf, in others projecting beyond the tip and forming a 

 short point or mucro, as in Tortula marginata ; or it may 

 form a long, transparent, hair-like point, as in Tortula muralis, 

 a moss very frequent on wall-tops. 



The leaves are placed spirally upon the stem and 

 branches, their arrangement being various, as \ or distichous 

 in Fissidens, \ or tristichous in Anccctangium, fths in 

 Pottia, or f as in Bryum. Their direction is variable, and 

 it is advisable to pay attention to this. Sometimes they are 

 crowded and imbricate (overlapping like tiles), as in Bryum 

 argenteum, common on walls ; or they may be spreading, as 

 in Tortula fallax, which may be seen on sandy or clayey 

 banks. In some species secund (curved to one side), as in 

 Dicranella heteromalla, frequent on wayside banks ; in others 

 remarkably recurved at the tips, or what is termed squarrose, 

 as in Hypnum squarrosum, to be found on heath lands and 

 in woods. 



When dry the direction of the leaves is often very dif- 

 ferent from that assumed when the plant is moist. Thus in 

 Bryum capillare the leaves are spreading when moist, but 

 much twisted when dry ; in Tortula spadicea much spread- 

 ing when moist, but closely imbricate when dry : but ex- 

 perience will soon show that these characters vary in 

 different species of moss. The margin of the leaf (fig. 

 5, 4) is sometimes plane, at others formed of a double row 

 of cells, and hence thickened, as in Tortula marginata / in 

 some cases entire, in others variously toothed. In some 

 species, Weissia controversa, for instance, it is involute 

 (rolled over towards the upper surface); in others re volute 

 (rolled over towards the lower surface), as in Tortula revoluta, 

 to be found on wall tops ; or the leaf may be rolled upon 

 itself from side to side, or convolute, as in the leaves sur- 

 rounding the base of the fruit-stalk of Tortula convoluta^ and 

 in some cases, as in Atrichum undulatum, the margin is 

 undulated. The leaf-surface is usually smooth, but in some 

 species, such as Thuidium tamariscinum (fig. 23, 2 ), it is 

 covered with minute projections, and is termed papillose. 



