omparable to _Ulva_ (Fig. 30, _C_, _D_); while others, among which is
the well-known Irish moss (_Chondrus_), form plants of considerable
size, with pretty well differentiated tissues. In such forms the outer
cells are smaller and firmer, constituting a sort of rind; while the
inner portions are made up of larger and looser cells, and may be
called the pith. Between these extremes are all intermediate forms.

They usually grow attached to rocks, shells, wood, or other plants,
such as the kelps and even the larger red seaweeds. They are most
abundant in the warmer seas, but still a considerable number may be
found in all parts of the ocean, even extending into the Arctic
regions.

[Illustration: FIG. 29.--_A_, a red seaweed (_Callithamnion_), of the
natural size. _B_, a piece of the same, x 50. _t_, tetraspores. _C_
i-v, successive stages in the development of the tetraspores, x 150.
_D_ I, II young procarps. _tr._ trichogyne. iii, young; iv, ripe spore
fruit. I, III, x 150. iv, x 50. _E_, an antheridium, x 150. _F_, spore
fruit of _Polysiphonia_. The spores are here surrounded by a case,
x 50.]

The methods of reproduction may be best illustrated by a specific
example, and preferably one of the simpler ones, as these are most
readily studied microscopically.

The form here illustrated (_Callithamnion_) grows attached to wharves,
etc., below low-water mark, and is extremely delicate, collapsing
completely when removed from the water. The color is a bright rosy
red, and with its graceful form and extreme delicacy it makes one of
the most beautiful of the group.

If alcoholic material is used, it may be mounted for examination
either in water or very dilute glycerine.

The plant is composed of much-branched, slender filaments, closely
resembling _Cladophora_ in structure, but with smaller cells
(Fig. 29, _B_). The non-sexual reproduction is by means of special
spores, which from being formed in groups of four, are known as
tetraspores. In the species under consideration the mother cell of
the tetraspores arises as a small bud near the upper end of one of
the ordinary cells (Fig. 29, _C_ i). This bud rapidly increases in
size, assuming an oval form, and becoming cut off from the cell of
the stem (Fig. 29, _C_ ii). The contents now divide into four equal
parts, arranged like the quadrants of a sphere. When ripe, the wall
of the mother cell gives way, and the four spores escape into the
water a