52 



EVOLUTION OF PLANTS 



the filament shows a distinction between base and apex. 

 The most specialized forms, e.g. Coleochsete (Fig. 10), 

 have the form of a flattened disk, and recall somewhat 



the structure found in 

 the simplest mosses. 



Within the Confer- 

 vacesB there is found a 

 similar advance in the 

 reproductive parts to 

 that described in the 

 Volvocinese. Some of 

 the lowest forms have as 

 yet shown only non-sex- 

 ual reproductive cells, 

 but it is not improbable 

 that sexuality will be 

 shown for all of them. 

 In these forms where 

 only non-sexual zo- 

 ospores have been ob- 

 served, they may be 

 either uniciliate or biciliate. These zoospores may arise 

 singly, by the escape of the whole of the contents of 

 a cell as a single zoospore ; or there may be a division 

 of the cell-contents into two or more parts which then 

 escape. When the zoospores are large (Fig. 9, D), they 

 sometimes have more than two cilia, but otherwise 

 resemble closely the typical Volvox cell. 



The simplest type of sexual reproduction among the 

 Confervacese consists in the formation of cells (gametes), 

 which differ from the zoospores only in being, as a rule, 

 smaller, but with no distinction of sex. These gametes 



ZBt. 



Fig. 8 (Confervacese). — A, a filament 

 of Microspora, composed of entirely 

 uniform cells ; B, part of a plant of 

 Cladophora, showing the branching 

 hahit ; C, a zoospore of Cladophora, 

 showing the two cilia, the eye-spot, 

 e, and the nucleus, n ; D, gametes, 

 or sexual cells, of Ulothrix, showing 

 the process of conjugation. (Fig.D, 

 after Dodel.) 



