LIFE HISTORIES OF ALG^E 251 



becomes organized into from one to eight motile spores 

 (zoospores, or swarm-spores), each with four cilia and a 

 red "eye-spot." The zoospore escapes by swimming 

 through a small opening in the cell-wall, attaches itself 

 by its ciliate end, and by a series of cell-divisions produces a 

 new filament like the one from which it came. In this 

 case it is evident that the zoospore, reproducing without 

 cell-fusion, is an asexual spore, and that the mother-cell 

 from which it came functioned as a sporangium. 



240. Sexual Reproduction. Other cells of the same 

 filament that produced the asexual zoospores, may, by 

 successive nuclear and cell-divisions, become divided into 

 as many as 16 to 64, or even more, independent motile cells, 

 each with a red "eye-spot," but with only two cilia. 

 Like the zoospores, they escape by swimming through an 

 opening in the wall of the mother-cell. Occasionally one 

 of them comes to rest and begins germination, but the 

 process never continues far enough to produce a new plant. 

 More commonly, two of these cells come together and fuse, 

 showing that they are, in reality, gametes. Since the 

 gametes are similar in size they are termed equal gametes, 

 or isogametes. The fusion of two isogametes is called 

 conjugation, to distinguish it from the fusion of un- 

 equal gametes; it is essentially the same as fertilization. 

 Whether or not reduction occurs, in the nuclear and cell- 

 divisions that result in the formation of the isogametes, 

 is not known. 



241. Germination. After the fusion of the gametes the 

 zygote at once begins to increase in size, but soon its cell- 

 wall becomes thickened, and then the protoplast divides 

 into a number of swarm-spores, each of which, when set 

 free, may develop into a new plant. 



