GROUP 



-THALLOPHYTA : ALG.E : CHLQROPHYCEJl. 



247 



somewhat pear-shaped in form, the more pointed end being colourless and 

 bearing four cilia and a pigment-spot ; the planogametes resemble the zoo- 

 gonidia but are proportionately smaller and have only two cilia. When the 

 zoogonidia come to rest, they secrete a cell-wall, and become attached by the 

 colourless end which forms the root cell of the developing filament. The piano- 

 gametes conjugate to form zygospores, but if they fail to conjugate they may 

 germinate independently, and they do so in the same manner as the zoogonidia 

 only the resulting filament is smaller. The zygospore grows and attaches itself 

 by its hyaline portion which developes into a root ; after a period of quiescence 

 its contents divide and are set free as 2-8 zoospores resembling the zoogonidia. 

 Fresh-water and marine. Principal genera: Ulothrix, Conferva, Hormidium, 

 Microspora, Binuclearia. 



FIG. 177. Ulothrix zona'.a : A 

 planogametes in different stages of 

 conjugation a-d ; B a zoogonidium. 

 (After Dodel-Port.) 



FIG. 176. Ulothrix zonata : A part of a fila- 

 ment from a cell of which planogametes are 

 escaping, the other cells having already emptied 

 themselves; B planogametes; C the process 

 of conjugation; D young zygospores; E mature 

 zygospore ; F germinating zygospore with 

 hyaline root ; G the contents of the embryonic 

 shoot of the sporophyte dividing to form zoo- 

 spores. 



Order 3. Chaetophoraceae. The body is attached by a basal cell, and is 

 erect or creeping ; it is filamentous and much branched, and usually bears long 

 hair-like outgrowths usually formed at the apex of the branches on the cessa- 

 tion of the merismatic activity of the apical growing-points ; the further elon- 

 gation of such branches is intercalary, being effected by the growth and division 

 of one of the cells near its base. 



Eeproduction by zoogonidia is known in almost all the genera ; the goni- 

 dangia are usually quite undifferentiated, though in some forms (e.g. Chroolepus) 

 they differ in form from the ordinary vegetative cells ; their development is 

 limited in some cases, either to terminal cells (Microthamnion), or to the cells 

 of the lateral branches (Draparnaldia) as distinguished from the main axis ; the 

 zoogonidia may be developed singly in the gonidangium (e.g. Chroolepus), in any 

 case the number does not exceed four ; the zoogonidia have frequently four cilia. 



Sexual isogamous reproduction is known in only a few genera (Stigeoclonium, 



