SECTION 17.] 



TIIALLOPHYTES. 



171 



set free is ready to gormiiiato. Fig. 505 represents several stages of the 

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

 this in one j)air 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 by 

 rig. 506, 507. Here the whole living contents of two individuals are in- 

 corporated into one spore, for a fresh start. A reproduction wliieh 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, iu 

 sexes. 



514. There are other Algfe 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. 508-574. 



515. Lichens, Latin Liclienes, are to be studied in the works of the 

 late Professor Tuekerman, 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 fructilication is named tlu^ Apolhecium. This is mainly 



Fia. 568. Early stage of a specie.s of Botrydiimi, a globose cell. 569, 570. Stages 

 of growth. 571. Full-grown ))lant, extended and ramified below in a root-liko 

 way. 57'2. A Vauoheria; single cell grown on into a nmch-branched thread; the 

 end of some branches enlarging, and the green contents in one {n) tiiere condensed 

 into a spore. 573. More magnified view of a, and the mature spore escnping. 

 574. Bryopsis plnmosa; apex of a stem with its branehleta; all the extensiou of 

 one cell. Variously raiignilied. 



