ON THE STUDY OF THE MOSSES. 195 



branched, as iu Hijpmim, (5). Id some of the tenxiiual-fruited mosses .it 

 branches by what are tei-med innovations ; these are es.tensions of the 

 stem, often arising at the top of the old stem, and such branching is 

 usually forked, each fork representing a year's gro%vth. This mode of 

 branching may be seen in many Bnjunu, and other mosses ; a convenient 

 example occurs in Grimmia pulviiiata, the little hoary, cushion-like 

 patches of which may be seen on wall-tops and thatch. 



The stem and branches are more or less densely clothed with 

 leaves, which are always simple, (undivided,) and vary in shape from 

 awl-shaped to round, the most frequent forms being lance-shaped, or 

 oval. The leaves vary in structure, but are usually formed of a single 

 layer of cells ; exceptions occur, as in Leucohryum ; in this case the 

 leaves are formed of three layers of cells. 



The cells forming the leaf assume a variety of foi-ms, but may be 

 referred to two types — I. Parenchymatous, (having the cells placed end to 

 end,) asm Fottia, &c., (16) ; 11. Prosenchymatous, (having cells ivhich over- 

 lap one unotJier at tlieir ends ; f t]xes,e have pointed ends, and are longer 

 than broad, as in Bryum, (15). The study of these leaf-cells is one of 

 great importance, as the generic and specific differences of many Mosses 

 are often made out by the character of the cells forming the leaf. 

 Among other forms assumed by cells we have round, as in Zygodon ; 

 quadrate, as in Fottia ; hexagonal, as in Tetraphis ; oblong, as in 

 Isothecium ; rhomboid, as in Bryum, &c. The cells at the base of the leaf 

 are frequently of different form from those of the upper part of the leaf, 

 and are often colourless and transparent. 



The centre of the leaf is often occupied by elongated cells, forming 

 what is called the nerve or midrib (6a. j This nerve is usually simple, but 

 may be forked as in Isotheciuminyurum ; or there may be two nerves, as in 

 Hypnwn triquetriim, common on marly banks ; or the leaves may be 

 nerveless, as in Hypnuui stcllatum. 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 foi"m a long transparent hair-like point, as in 

 Tortula iiiuralis, 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 Fissiden-s, J or tristichous 

 in Ancectaiigiuni, 2-5ths in Fottia, or g 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 argenteuvi^ 

 common on walls ; or they maybe 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 sqtiarrosum, to be found on heath lands and in woods. 



When dry the direction of the leaves is often very different 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 spreading when moist, but closely imbricate when diy ; 



