513. Desmids and Diatomes, which are microscopic one-celled plants of the same class, conjugate in the same way, as is shown in a Closterium by Fig. 566, 567. Here the whole living contents of two individuals are incorporated into one spore, for a fresh start. A reproduction which costs the life of two individuals to make a single new one would be fatal 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 Algæ 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.