Heterokontce 249 



anterior extremity of the zoogonidium and are generally carried in 

 opposite directions. Each zoogonidium usually possesses several 

 parietal chromatophores (vide fig. 121 C). 



Aplanospores are also frequently formed, usually one in each 

 cell and possessed of thick cell-walls. 



Sexual reproduction occurs by the fusion of isogamous plano- 

 gametes which most probably resemble the zoogonidia in the 

 possession of two unequal cilia 1 . It is only very recently (1898) 

 that the second short cilium was demonstrated, up to that time 

 the zoogonidia and gametes of these AlgaB having been described 

 as possessing only one cilium. The second short cilium is usually 

 carried in a backward direction, pressed closely against the body of 

 the cell. 



The Heterokontse contains only a limited number of genera. 

 None of the Flagellate series 'Chloromonadales' have been observed 

 from the British Islands, and all the remaining genera belong to 

 the Confervales. Chlorobotrys Bohlin, which that author referred 

 to the Chloromonadales, is strictly algal in character and belongs 

 to the Tribonemaceae. 



Order I. CONFERVALES. 



In this order the cells are entirely algal in character, thus 

 differing from those of the Flagellate order Chloromonadales. 



The plants are unicellular, multicellular, or crenocytic, and the 

 cell-walls are sometimes very thick. Each cell contains several or 

 many discoidal chromatophores, with a parietal disposition, and 

 from which pyrenoids are absent. 



The reproduction is by zoogonidia and isogamous planogametes, 

 the former possessing a pair of unequal cilia. 



The order is divided into three families : 



Family 1. Chlorotheciacece. Unicellular or colonial; cells small, 

 often attached by basal stalks, uninucleate, with one or many chro- 

 matophores. 



Family 2. Tribonemacece. Unicellular or filamentous ; cells often 

 elongate, uninucleate or subccenocytic, with several or many chromato- 

 phores ; cell-walls firm and thick. 



Family 3. Botrydiacece. Plant-body large and coanocytic, fixed, 

 with well-developed organs of attachment ; chromatophores numerous. 



1 Gametes have been described in certain genera with two equal cilia, but such 

 statements are not established facts and require re-investigation. 



