SECTION 17.] 



THALLOPHYTES. 



171 



set free is ready to germinate. Fig. 565 represents several stages of the 

 conjugating process, which, however, would never be found all together like 

 this in one pair of threads. 



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 bj 

 Fig. 566, 567. Here the whole living contents of two individuals are in- 

 corporated 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 divi- 

 sion 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. 



674 



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 



Fio. 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 pluraosa; apex of a stem with its branchlets; all the extension of 

 one cell. Variously magnified. 



