INTRODUCTION. 9 



most cases they soon separate. In those species which 

 possess a sculptured cell- wall the neAv semicells are ;if 

 first quite smooth, the markings only beginning to 

 appear as the semicells approach maturity. In those 

 species which have an incised outline the incisions 

 begin to form lono- before the semicells have attained 



t~> 



half their full size. Thus, in all Desmids one semicell 

 is younger than the other. Division is most active in 

 late spring or early summer according to the con- 

 ditions of the climate. 



ASEXUAL REPEODUCTION takes place by means of 

 apla/nospores in certain species of Desmids. Each 

 aplanospore is formed by the rejuvenescence of the 

 entire contents of a cell, a new cell-w^all being 

 developed round the spore. This method of repro- 

 duction is of exceedingly rare occurrence, having only 

 been seen by Wallich and Turner in Spondylosium 

 t/itcitx and by ourselves in Hyalotheca neijlcrtn. Ealfs 

 most probably noticed spores of this nature in 

 D< '* midiuTi i /S'v tv 1 1 iz ii. 



Archer's account of the occurrence of zoospores in 

 a ' Docidium' must have resulted from a mistaken 

 observation, as all evidence tends to prove the entire 

 absence of ciliated motile spores from the order Con- 

 jugate. It is most probable that some parasitic 

 organism was present in the ' DoruJutniJ which gave 

 origin to the zoospores observed by him. 



SEXUAL REPRODUCTION of an exceedingly degenerate 

 type occurs in all the genera of Desmids. It takes 

 place by the conjugation of isogamous gametes, i. e. by 

 the union of precisely similar gametes. The ordinary 

 vegetative cells themselves become the gametangia, 

 and as the gametes possess no cilia they are known as 

 aplanogametes . 



Two cells of any one species having become approxi- 

 mated, they arrange themselves parallel to each other 

 or inclined at various angles according to the genus to 

 which they belong. From the middle of the contiguous 

 sides, usually the sinus, a short connecting-tube is 



