8 Introduction 



Algae should always be examined in the living state whenever 

 possible, as some of them are more easily determined when alive. 

 Many of the Desmidiaceae and Bacillariese, however, especially those 

 with characteristic surface markings, can only be determined with 

 precision from the dead empty cells or semicells. 



In preserving Alga? for future examination several fluids may 

 be used. In studying the structure of the cell-contents a 2 4 / o 

 formalin solution is best for subsequent staining, etc. This is 

 almost equalled by a dilute solution of picric acid. But if only 

 the cell-outlines and the structure of the cell-wall are required 

 then there is no better preservative than a 4 / solution of potas- 

 sium acetate (containing a trace of copper acetate). An equal 

 volume of this solution added to the water in which the Alga3 are 

 living is quite sufficient. A very instructive paper has been 

 published by Pfeiffer R. v. Wellheim 1 on the methods of prepara- 

 tion, staining, etc., of algse, in which chromacetic acid is largely 

 recommended for preserving them. Certain reagents such as a 

 solution of iodine, methylene blue, hsematoxylin, ammonia-car- 

 mine, etc., are almost essential to all students of Algse. A 2 / 

 cocaine solution is also very useful for the observation of the cilia 

 of motile forms. 



Some Algae can be preserved by drying, either on paper or on 

 slips of mica, but they are always better for purposes of future 

 examination when preserved in a fluid medium than when dried. 

 The only Algse that preserve well by drying are the Myxophycese. 

 These Algse on soaking out again in water almost regain their 

 original freshness, and, moreover, retain their original bright 

 colours. 



Algse are best mounted in the fluid in which they have been 

 preserved, and the best varnish with which to seal them up is 

 gold-size. Everyone who has had any experience of fluid mounts, 

 however, knows quite well that if his specimens keep for a long 

 time it is due more to good fortune than to any other cause. 

 Many fluid mounts, even the best ones, frequently begin to dry up 

 by the formation of air-bubbles in the centre of the slide, which 

 gradually extend towards the periphery. The only explanation of 

 this is the porous nature of the thin coverslip. 



In examining Algae, in following out their life-history, and in 



1 F. Pfeiffer R. v. Wellheim, ' Preparation der Siisswasseralgen,' iu Pringsheirn's 

 Jahrbuch. t'iir wissensch. Botan. Bd xxvi, Heft 4, Berlin, 1894. 



