fiONATOZYGON. 29 



Sul .-family I. SAGGODERHLE. 



IN tins sub-family the cell-wall is unsegmented and 

 entirely without pores. It is absolutely continuous, the 

 newer half being indistinguishable from the older half. 

 There is no fixed point at which division takes place 

 (except in a few species of Cylindrocystift), and the 

 young semicells are developed obliquely. 



Tribe 1. GTONATOZYG.E. 



The two genera included in this tribe are consider- 

 ably removed from most other Desmids. The cells are 

 long, more or less cylindrical, and are united by their 

 apices to form very fragile filaments of variable length. 

 A verv slight disturbance will cause the filaments to 



/ * ' 



dissociate into their individual cells, each of which 

 then lives an independent existence. The cell-wall 

 consists of two layers, the inner one being hyaline and 

 structureless and the outer one being generally diffe- 

 rentiated so as to give rise to the minute prominences 

 and delicate spines which are characteristic of these 

 plants. Conjugation only takes place between cells 

 which have become free. 



Genus 1. GONATOZYGON De Bary, 185(3. 



De Bary, in Hedwigia, 1856, p. 105 ; Conj. 1858, p. 26. 

 Cooke, Brit. Desm. 1886, p. 2. 



Cells cylindrical or narrowly snbfnsiform, 10-20 

 (rarely 40) times longer than their diameter, not con- 

 stricted, truncate, generally slightly dilated and often 

 snbcapitate at the apices ; usually remaining attached 

 to each other in filaments of variable length, which 

 readily dissociate into the separate cells when disturbed, 

 and always before conjugation ; during conjugation 

 sometimes geniculate. Chloroplasts two, or sometimes 

 only one, axile, generally undulate and rather narrow, 

 containing from four to sixteen equidistant pyrenoids. 



Zygospore globose and smooth. 



