they grow.

[Illustration: Fig. 555. Magnified section through a fertile conceptacle
of Rockweed, showing the large spores in the midst of threads of cells.
556. Similar section of a sterile conceptacle, containing slender
antheridia. From Farlow's "Marine Algae of New England."]

509. The hollow and inflated places in the Fucus vesiculosus or Rockweed
(Fig. 554) are air-bladders for buoyancy. The fructification forms in
the substance of the tips of the frond: the rough dots mark the places
where the conceptacles open. The spores and the fertilizing cells are in
different plants. Sections of the two kinds of conceptacles are given in
Fig. 555 and 556. The contents of the conceptacles are discharged
through a small orifice which in each figure is at the margin of the
page. The large spores are formed eight together in a mother-cell. The
minute motile filaments of the antheridia fertilize the large spores
after injection into the water: and then the latter promptly acquire a
cell-wall and germinate.

510. The Florideae or Rose-red series of marine Algae (which, however, are
sometimes green or brownish) are the most attractive to amateurs. The
delicate Porphyra or Laver is in some countries eaten as a delicacy, and
the cartilaginous Chondrus crispus has been largely used for jelly.
Besides their conceptacles, which contain true spores (Fig. 560), they
mostly have a fructification in _Tetraspores_, that is, of spores
originating in fours (Fig. 559).

[Illustration: Fig. 557. Small plant of Chondrus crispus, or Carrageen
Moss, reduced in size, in fruit; the spots represent the fructification,
consisting of numerous tetraspores in bunches in the substance of the
plant. 558. Section through the thickness of one of the lobes,
magnified, passing through two of the imbedded fruit-clusters. 559. Two
of its tetraspores (spores in fours), highly magnified.]

[Illustration: Fig. 560. Section through a conceptacle of Delesseria
Leprieurei, much magnified, showing the spores, which are single
specialized cells, two or three in a row.]

[Illustration: Fig. 561. A piece of the rose-red Delesseria Leprieurei,
double natural size. 562. A piece cut out and much magnified, showing
that it is composed of a layer of cells. 563. A few of the cells more
highly magnified: the cells are gelatinous and thick-walled.]

511. The Grass-green Algae sometimes form broad membranous fronds, such
as those of the common Ulva of the sea-shore, but mos