!<>(> UHITISH DESMIDIACEJ-;. 



Tribe 4. CLOSTEKIE.K. 



The cells are elongate, usually curved and commonly 

 attenuated towards each extremity. There is no median 

 constriction, and the plants are circular in cross- 

 section. The place of division is regularly situated in 

 the middle region of the cell, and the cell-wall gener- 

 ally possesses pores. They are symmetrical about one 

 longitudinal plane. 



Germs 8. ROYA West & G. S. West, 1890. 



West & G. S. West, New and Int. Freshw. Alg. 1896, p. 152. 

 Nordst. Index Desm. 1896, p. 280. 



Cells very slightly arcuate, almost exactly cylindri- 

 cal, scarcely attenuated towards the extremities, apices 

 subtruncate or obtusely rounded; cell-wall smooth and 

 colourless ; one chloroplast in each cell, generally with 

 a small excavation in the middle of the concave side in 

 which the nucleus is lodged ; the extremities of the 

 chloroplast are convex and extend almost to the ex- 

 treme ends of the cell, there not being any apical 

 vacuoles nor moving corpuscles ; pyrenoids 4-14 in a 

 single series. 



The late Dr. J. Roy first pointed out the characters of this 

 genus, remarking under " Closterium Pseudoclusterium " that 

 "this curious little species forms one of a small group of 

 which Cl. olitiisiim Breb. may be taken as the type. They do 

 not accord well witli Clotiterium, and undoubtedly should be 

 placed in a new genus." As we had been fully convinced for 

 some time that these plants could not remain in the genus 

 Closterium, we described the genus Boi/u in 1896. 



Poi/a is a well-marked genus which can be readily dis- 

 tinguished from either Penhim or Closterium; in fact the 

 differences between Roya and those genera are far more 

 striking than the differences between those genera themselves. 



We think the genus sufficiently distinct from Clout erinm, 

 by reason of the unbroken chloroplast which extends to the 

 extremities of the cells, leaving no room for apical vacuoles. 

 Specimens of certain species of Closterium are occasionally 

 without apical vacuoles containing- moving- corpuscles, but 

 they have always sufficient space for the lodgment of a 



