MORPHOLOGY OF MOSSES

The following discussion of the morphology of the typical moss is a brief discussion of those parts of the gametophyte and the sporophyte of which an understanding is necessary in order to identify the common Mount Rainier mosses. In this section the gametophyte structures will be considered first.

THE MOSS GAMETOPHYTE

Most of the mosses produce spores from which the gametophyte generation, or the familiar leafy plant, develops. An example of a fairly common Mount Rainier moss which reproduces asexually, is the Aulacomnium androgynum which produces brood bodies in special clusters, or pseudopodia at the tip of the plant. Another common moss, Pohlia annotina, bears bulbils or gemmae among the leaves.

Spores of mosses, such as the Polytrichum spores shown, are usually sperical and smooth, although sometimes roughened. In color they are green due to the chlorophyll which they contain.

Protonema are produced when the spore germinates. These are tiny green threads which soon grow into a thick, felty mass. They are divided transversely by cross walls. At intervals on the protonema buds arise, and these develop into the leafy shoots. There are no true roots, but rhizoids or absorbing structures.

The leaves are usually one cell in thickness except at the midrib and occasionally the margin. Leaves are very important in the identification of mosses. The drawings are meant to illustrate some of the most important characters of leaves.

The shape of the leaf is one of the most easily recognized features. An ovate leaf is one which is egg-shaped. Obovate refers to one unusually longer but at least with the base of the leaf narrower than the middle. An oblong leaf is at least half again as long as broad. An intermediate type is called ovate-oblong. Lanceolate is of spearhead shape with a long, tapering, sharp point and five to six times as long as broad. Linear leaves are long and slender. Finally, spathulate leaves are broader above than at the base, and the tip is rounded.

Next to shape, the most important leaf feature is the midrib, or costa, which may be single, double, forked, or completely lacking (ecostate leaf). In the Polytrichaceae, abundantly represented in the Park the costa bears on its upper surface longitudinal strips of tissue (lamellae). When the costa does not run to the tip, it is a vanishing costa; when it forms the tip, it is percurrent; when it extends beyond the tip, it is excurrent.

PLATE III. GAMETOPHYTE MORPHOLOGY

TYPES OF LEAVES AS TO SHAPE
1 - ovate. 2 - obovate. 3 - oblong. 4 - lanceolate. 5 - linear. 6 - spatulate.

TYPES OF COSTAE AMONG MOSSES
1 - percurrent. 2 - excurrent. 3 - double. 4 - vanishing below the apex of the leaf.

The apex of the leaf is mucronate if it has a stout, abrupt point; cuspidate if the leaf has a long, stiff point; acuminate if there is a gradually tapering, narrow point; acute if there is a short, sharp point. A muticous leaf, such as those of the Andreacaceae, has no point. The tip of the leaf is termed falcate when it curves to one side, and circinate when it forms almost a complete circle.

Leaf margins that are wavy are flexuose, and when the leaf is curled or contorted when dry, it is crisp. The edge of the leaf may be entire or unbroken or serrate. In the Mniums the cells of the leaf margin are differentiated and the leaf is said to be bordered.

PLATE IV. GAMETOPHYTE MORPHOLOGY

TYPES OF LEAF APICES
1 - mucronate. 2 - cuspidate. 3 - acuminate. 4 - acute. 5 - muticous. 6 - falcate. 7 - circinate.

TYPES OF LEAF MARGINS
1 - flexuose. 2 - crisp. 3 - entire. 4 - serrate.

Another very important detail to watch for in identifying mosses is the arrangement of the leaves on the stems. Where leaves lie flat in two rows on each side of the stem, they are complanate. If they stand close to the stem, erect; but if they diverge 45 degrees or less, they are patent. On the other hand, a divergence of over 45 degrees makes them spreading. If such spreading leaves are turned back slightly, they are then squarrose, and if turned strongly to one side, they are secund. Again, the leaves may closely overlap like shingles on a roof, and are then termed imbricated. Some of the Polytrichaceae, as Polytrichum piliferum, when dry show an appressed condition of the leaves.

PLATE V. GAMETOPHYTE MORPHOLOGY

TYPES OF LEAVES AS TO ARRANGEMENT
1 - erect. 2 - spreading. 3 - secund. 4 - squarrose.

TYPES OF LEAVES AS TO ARRANGEMENT
1 - complanate. 2 - patent. 3 - imbricated. 4 - appressed.

The Method of Branching is a means of distinguishing between the two great moss groups - the Acrocarpous and the Pleurocarpous mosses. The former have the stem erect and usually not branched (Typical acrocarpous; if branched, then slightly so by forking or distichous such as Bartramia pomiformis.

The Pleurocarpous mosses have stems prostrate, or ascending, rarely erect, and usually much branched. If the branches are quite regularly arranged on opposite sides of the stem, then the plant is regularly pinnate. When the branches come off at varying angles, the plant is irregularly pinnate.

PLATE II. GAMETOPHYTE MORPHOLOGY

METHODS OF BRANCHING AMONG ACROCARPOUS MOSSES
1 - typical. 2 - distichous.

METHODS OF BRANCHING AMONG PLEUROCARPOUS MOSSES
1 - typical. 2- regularly pinnate. 3 - irregularly pinnate.

Introduction continued...