INTRODUCTION. 



AT the period when the researches hereafter to be detailed 

 were undertaken with a view to publication, viz,, in the 

 spring of the year 1840, no department of Cryptogamic 

 Botany was in so unsatisfactory and obscure a state as that 

 of the freshwater Algce ; the works and memoirs, compara- 

 tively few in number, which had then appeared, either in this 

 country or on the Continent, abounding with descriptions 

 incomplete, inaccurate, or repetitions of the same productions 

 and facts under different forms and appearances. 



That such should have been the case is not so surprising, 

 when the minuteness of the objects composing the majority 

 of this fertile class of Nature's exhaustless works is considered 

 (the individual parts of many of them being more slender 

 than the human hair), and when, also, the imperfection of 

 the microscopic instruments until recently employed in their 

 investigation, and their changing and fragile character, are 

 taken into account, these circumstances rendering a patient 

 and long-continued study of them necessary. It nevertheless 

 must be regarded as somewhat remarkable, that a field so 

 rich in discovery and of such high interest, until very lately, 

 should have been so little explored ; and that such would 

 have been the case, was certainly not in accordance with the 

 expectations of Vaucher when he penned the following 

 remarks in the introduction to his eloquent and admirable 



