Page:Midland naturalist (IA midlandnaturalis01lond).pdf/235

 branched, as in Hypnum, (5). In some of the terminal-fruited mosses it branches by what ave termed innovations; these are extensions of the stem, often arsing at the top of the old stem, and such branching is usually forked, each fork representing a year's growth. This made of branching may be seen in many Bryums, and other mosses; a convenient example occurs in Grimmia pulvinata, 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 Leucobryum; in this case the leaves are formed of three layers of cells.

The cells forming the leaf assume a variety of forms, but may be referred to two types—I. Parenchymatons, (having the cells placed end to end,) as in Poittia, &c., (16); II. Prosenchymatous, (having cells which overlap one another at their ends;) these 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 lave round, as in Zygadon; quadrate, as in Pottia; 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.) This nerve is usually simple, but may be forked as in Isothecium myaram; or there may be two nerves, as in Hypnum triquetrum, common on marly banks; or the leaves may be nerveless, as in Hypnum stellatum. The nerve is of variable length, in same cases vanishing below the lip of the leaf, in others projecting beyond the tip and forming a short point or mucro, as in Tortula marginata; or it way form a long transparent hairlike 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 ½ or distichous in Fissidens, ⅓ or tristichous in Anæctangium, 2-5ths in Poittia, or ⅝ 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 second, (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 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 spadicca much spreading when moist, but closely imbricate when dry;