DESMIDIOI. 251 



ENGLAND.- -Loughrigg, AYestmoreland ! Adel Bou, 

 W. Yorks, and Strensall, X. Yorks (TF. B. Turn.). 

 War wicks ! Thursley Common, Surrey ! New Forest, 

 Hants ! Dartmoor, Devon (Harris). Cornwall ! (Mar- 



WALES. Capel Curig (Cooke d* Wills) and Bettws-y- 

 coed (Roy), Carnarvonshire. 



SCOTLAND. General ; zygospores from Slewdrum in 

 Birse (Roy & Biss.). 



IRELAND. Donegal (zygospores from Dungloe) ! 

 Achill Isle. Mayo ! < la I way (zygospores from Ballyna- 

 binch) ! Kerry ! Dublin and Wicklow (Arch.). 



(icon r. Distribution.- -France. < Germany. Switzer- 

 land. Galicia and Austria. Italy. Xorway. Sweden. 

 Denmark. Finland. Poland. X. and S. Russia. 

 India. Ceylon. Australia. Dnited States. Guiana. 

 Colombia. Brazil. Para^uav. 



O 



D. ctflin<lririnn. although one of the most frequent of all the 

 species of the genus, and at the same time fairly widely distri- 

 buted, is rarely abundant. It is one of the largest species, and 

 is easily recognised by its short broad cells with their simple 

 apical attachment, and the elliptical vertical view, with a small 

 mamilla at opposite poles. Raciborski has seen a triradiate 

 form of the species, in which the end view has three equidistant 

 mamillse on the nearlv circular outline. 



V 



The most peculiar and important fact about D. cylindricum 

 is that its zygospore is formed within one of the conjugating 

 cells. This is the only known Desmid in which the reproduction 

 is normally of such a high type that the conjugating cells can be 

 distinguished definitely as and $ gametangia, although the same 

 thing is known to occur occasionally as an abnormality in Hyalo- 

 tlicca dissiliens. It is supposed that in this phenomenon the 

 method of reproduction of the immediate ancestors is disclosed, 

 i. e. the ancestors of the Desmidieae were filamentatous, and had 

 a well differentiated type of sexual reproduction. In the evolu- 

 tion of the Desmidiea3 the acquisition of the unicellular condition 

 and the development of a highly complicated morphological 

 structure has gone hand in hand with the degeneration of the 

 form of reproduction, so that the former high type only remains 

 in the above-mentioned cases. 



