atal to the species if there were not a provision for multiplication by
the prompt division of the new-formed individual into two, and these
again into two, and so on in geometrical ratio. And the costly process
would be meaningless if there were not some real advantage in such a
fresh start, that is, in sexes.

[Illustration: Fig. 568. Early stage of a species of Botrydium, a
globose cell. 569, 570. Stages of growth. 571. Full-grown plant,
extended and ramified below in a root-like way. 572. A Vaucheria; single
cell grown on into a much-branched thread; the end of some branches
enlarging, and the green contents in one (_a_) there condensed into a
spore. 573. More magnified view of _a_, and the mature spore escaping.
574. Bryopsis plumosa; apex of a stem with its branchlets; all the
extension of one cell. Variously magnified.]

514. There are other Algae of the grass-green series which consist of
single cells, but which by continued growth form plants of considerable
size. Three kinds of these are represented in Fig. 568-574.

515. =Lichens=, Latin _Lichenes_, are to be studied in the works of the
late Professor Tuckerman, but a popular exposition is greatly needed.
The subjoined illustrations (Fig. 575-580) may simply indicate what some
of the commoner forms are like. The cup, or shield-shaped spot, or knob,
which bears the fructification is named the _Apothecium_. This is mainly
composed of slender sacs (_Asci_), having thread-shaped cells
intermixed; and each ascus contains few or several spores, which are
commonly double or treble. Most Lichens are flat expansions of grayish
hue; some of them foliaceous in texture, but never of bright green
color; more are crustaceous; some are wholly pulverulent and nearly
formless. But in several the vegetation lengthens into an axis (as in
Fig. 580), or imitates stem and branches or threads, as in the
Reindeer-Moss on the ground in our northern woods, and the Usnea hanging
from the boughs of old trees overhead.

[Illustration: Fig. 575. A stone on which various Lichens are growing,
such as (passing from left to right) a Parmelia, a Sticta, and on the
right, Lecidia geographica, so called from its patches resembling the
outline of islands or continents as depicted upon maps. 576. Piece of
thallus of Parmelia conspersa, with section through an apothecium. 577.
Section of a smaller apothecium, enlarged. 578. Two asci of same, and
contained spores, and accompanying filaments; more