12 mUTISH DESMJDIAl'K.E. 



They constitute unquestionably the family of Con- 

 jugates which has attained a maximum state of 

 specialization with regard to complexity of morpho- 

 logical characters, accompanied by the loss of the 

 filamentous condition and the degeneration of sexual 

 differences. It is a notable fact that Desmidium ryliii- 

 ilricinn- is the only known Desmid in which the 

 zygospore remains in one of the conjugating cells 

 (presumably the female), and the occasional reversion 

 to this type in Hyalotheca dixsilims goes far to prove 

 that in all probability this was their ancestral type of 

 conjugation, a type which still exists in the Zygne- 

 macea3, but which the Desmidiaceas have lost except 

 for the lingering remains of it Avhich are found in 

 Destnidium cylindricum. Another fact testifying to 

 this degeneration is the secondary assumption of the 

 filamentous condition by about eio-ht o-enera and several 



*/ o o 



individual species of other genera. The recent discovery 

 of that remarkable Conjugate Debarya desmidioides 

 has also added still further evidence with regard to 

 the evolution of Desmids from ancestral filamentous 

 Conjugates. 



OCCURRENCE AND DISTRIBUTION. Desmids occur from 

 sea-level to just below the snow-line. A few species 

 have been found in water that was somewhat brackish, 

 but only in small numbers. Everything seems to 

 indicate that they have but a precarious existence in 

 such a medium ; indeed, it is not only possible but 

 probable that they have been introduced in such situa- 

 tions by aquatic birds. As they are usually free- 

 floating plants they are not as a rule found in 

 running waters. They are most abundant in perma- 

 nent shallow pools and the quiet, sheltered recesses of 

 small lakes. Some species can always be found in the 

 hollows of peaty moors, whether natural or artificial, if 

 of sufficient age. They are usually most abundant both 

 with regard to number of species and individuals in the 

 pools, tarns, and lakes of rocky districts. In flat dis- 

 tricts like the fens they are few in number, especially 



