ery
numerous, and the plant may reach a length of a metre or more. The
branches are flattened and leaf-like, the centre traversed by a
thickened midrib. The end of the growing branches is occupied by a
transversely elongated pit or depression. The growing point is at the
bottom of this pit, and by a regular forking of the growing point the
symmetrical branching of the plant is brought about. Scattered over
the surface are little circular pits through whose openings protrude
bunches of fine hairs. When wet the plant is flexible and leathery,
but it may become quite dry and hard without suffering, as may be seen
when the plants are exposed to the sun at low tide.

The air bladders are placed in pairs, for the most part, and buoy up
the plant, bringing it up to the surface when covered with water.

The interior of the plant is very soft and gelatinous, while the outer
part forms a sort of tough rind of much firmer consistence. The ends
of some of the branches (Fig. 26, _A_, _x_) are usually much swollen,
and the surface covered with little elevations from which may often be
seen protruding clusters of hairs like those arising from the other
parts of the plant. A section through one of these enlarged ends shows
that each elevation corresponds to a cavity situated below it. On some
of the plants these cavities are filled with an orange-yellow mass; in
others there are a number of roundish olive-brown bodies large enough
to be easily seen. The yellow masses are masses of antheridia; the
round bodies, the ooegonia.

If the plants are gathered while wet, and packed so as to prevent
evaporation of the water, they will keep perfectly for several days,
and may readily be shipped for long distances. If they are to be
studied away from the seashore, sections for microscopic examination
should be mounted in salt water (about 3 parts in weight of common
salt to 100 of water). If fresh material is not to be had, dried
specimens or alcoholic material will answer pretty well.

To study the minute structure of the plant, make a thin
cross-section, and mount in salt water. The inner part or pith is
composed of loosely arranged, elongated cells, placed end to end,
and forming an irregular network, the large spaces between filled
with the mucilaginous substance derived from the altered outer walls
of these cells. This mucilage is hard when dry, but swells up
enormously in water, especially fresh water. The cells grow smalle