INTRODUCTION.

At the period when the researches hereafter to be detailedwere undertaken with a view to publication, viz., in thespring of the year 1840, no department of CryptogamicBotany was in so unsatisfactory and obscure a state as thatof the freshwater Alga ; the works and memoirs, compara-tively few in number, which had then appeared, either in thiscountry or on the Continent, abounding with descriptionsincomplete, inaccurate, or repetitions of the same productionsand 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 majorityof this fertile class of Nature ’s exhaustless works is considered(the individual parts of many of them being more slenderthan the human hair), and when, also, the imperfection ofthe microscopic instruments until recently employed in theirinvestigation, and their changing and fragile character, aretaken into account, these circumstances rendering a patientand long-continued study of them necessary. It neverthelessmust be regarded as somewhat remarkable, that a field sorich in discovery and of such high interest, until very lately,should have been so little explored; and that such wouldhave been the case, was certainly not in accordance with theexpectations of Yaucher when he penned the followingremarks in the introduction to his eloquent and admirable