LIVERPOOL FRESH WATER ALG^E. 273 



of the rocks and subsoils, especially the sandstones and 

 clays, are important factors in the determination of this 

 richness. On the Cheshire side one may walk for miles 

 and pass scores of fields and find one or more ponds in 

 each field. Now it will readily be expected that many 

 forms of Algse will occur frequently, and particularly the 

 simpler forms, and such is the case, for many of the 

 unicellular Algae are met with in almost every gathering 

 and at almost any time of the year. 



The difficulties of certifying to the specific characters 

 of many of these forms are great indeed ; form, size, con- 

 dition, still or active, singly or in mass, vary with many 

 changing conditions and many times in a short period. 

 Be it remembered that the very simplicity of structure of 

 these simple plants makes them susceptible and respon- 

 sive to relatively feeble changes in the conditions that are 

 at once remarkable and surprising to students of higher 

 forms of plant life. Of course active cell structures of the 

 higher plants are acted and reacted upon by slight changes 

 of conditions, e.g., food, temperature, light, moisture, etc., 

 but the changes largely pass unnoticed as they require 

 longer time, and the influence is evident only in the 

 result. 



These minute plants all require microscopical examina- 

 tion to determine even the most superficial characters, 

 whilst the characters determining specific differences often 

 require close, skilful and patient investigation. Again and 

 again conclusions have to be postponed for want of stages 

 in the life history or development of the plant ; the factor 

 of reproduction is so important to a true classification that 

 much material has frequently to be cast aside as incom- 

 plete for determination. As in other groups of organisms 

 careful selection and cultivation of the particular forms to 

 determine the true life history is all important, so in the 



