INTRODUCTION. 21 



We would also impress upon future workers the 

 desirability of examining any district they are study- 

 ing at all periods of the year, and for more than one 

 year; this applies more especially to those resident in 

 or near such a district. 



We have been very considerably helped in our 

 investigation of the distribution of these plants in some 

 of the remote districts by three grants from the 

 Government Grant Committee of the Royal Society. 



/ i/ 



Dr. Otto Nordstedt of Lund, Sweden, has very 

 kindly placed at our disposal the valuable Desmid 

 material he collected when he visited Britain ; this 

 embraces many scores of gatherings. The material 

 has been very useful, as such a noted and experienced 

 algologist knew exactly how and where to collect. We 

 believe that this work will show that considerably 

 more is known concerning the Desmids of the British 

 Isles than of those of any other country. 



We have arranged the species of the larger genera 

 in such a way as to facilitate their study. 



The total number of Desmids known in the world in 

 1839 was about 90 species, in 1861 this had increased 

 to about 300, in 1889 to about 1200, and in 1902 to 

 about 2000. The number of species known to occur 

 in the British Isles is approximately (390. 



