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...
