102 Chlorophycece 



and sexual reproduction is either by isogamous planogametes or 

 by well-differentiated heterogamous gametes. 



The order includes the following three families : 



Family 1. Cladophoracece. Thallus large, branched, incompletely 

 septate, usually attached. Segments large with numerous small 

 chloroplasts, each with a pyrenoid. 



Family 2. Pithophoracece. Thallus similar to that of the Clado- 

 phoracese, but distinguished by the formation of barrel-shaped and 

 fusiform asexual resting-spores. 



Family 3. Sphceropleacece. Filaments simple, composed of elongated 

 coenocytes. Sexual reproduction heterogamous; plants monoecious; 

 oogonia with many non-motile oospheres ; fertilization within the 

 oogonium. 



Bohlin has recently proposed to transfer the families Clado- 

 phoracese (including the PithophoraceaB) and Spha3ropleacese to 

 the order Siphoneas, placing them next the Valoniacese. In this 

 he is followed by Blackman and Tansley, but it yet remains to 

 be shown how far this change is justified. The thallus of the 

 Cladophorales is much more septate than that of the Siphonea3 

 and the branching is of a different nature. 



Family 1. CLADOPHORACE^. 



The thallus is large, filamentous and incompletely septate, each 

 segment being a ccenocyte. The filaments usually have a basal 

 organ of attachment, and in the genus Cladophora are much 

 branched. The growth of the thallus is apical in Cladophora but 

 intercalary in the other genera. 



There are several nuclei in each segment of the thallus and 

 either one reticulate, parietal chloroplast or a large number of 

 separate chloroplasts, each with a pyrenoid. 



Asexual reproduction takes place in Cladophora and Chceto- 

 morpha by zoogonidia which are produced in large numbers in the 

 mother-cell and escape either by a terminal or lateral pore. 'Cysts' 

 are also produced in Rhizodonium, each one being a kind of large 

 thick-walled akinete formed from a single segment. 



An isogamous sexual reproduction occurs in Cladophora, the 

 conjugation of the planogametes resulting in a zygospore which 

 germinates directly without rest. 



Very little, if any, mucus is secreted by these Algae and they 

 always feel rough and crisp. The absence of a mucous outer coat 



