INTRODUCTION. XI 



tation of the waters. The sea, in no climate from the po- 

 lar circle to the equator, is altogether free from Algae, 

 though they abound on some shores much more than on 

 others, a subject which will come particularly under notice 

 when we speak of the distribution of their several tribes. 

 Species abound likewise in fresh water, whether running or 

 stagnant, and in mineral springs. The strongly impreg- 

 nated sulphureous streams of Italy, the eternal snows of 

 the Alps and arctic regions, and the boiling springs of 

 Iceland, have each their peculiar species ; and even che- 

 mical solutions, if long kept, produce Alga?. Very few, 

 comparatively, inhabit stations which are not submerged 

 or exposed to the constant dripping of water ; and, in all 

 situations where they are found, great dampness, at least, 

 is necessary to their production. 



Thus extensively scattered through all climates, and ex- 

 isting under so many varieties of situation, the species are, 

 as one would naturally suppose, exceedingly numerous, 

 and present a greater variety in form and size than is ob- 

 servable in any other tribe of plants whose structure is so 

 similar. Some are so exceedingly minute as to be wholly 

 invisible, except in masses, to the naked eye, and require 

 the highest powers of our microscopes to ascertain their 

 form or structure. Others, growing in the depths of the 

 great Pacific Ocean, have stems which exceed in length 

 (though not in diameter) the trunks of the tallest forest 

 trees ; and others have leaves that rival in expansion those 

 of the Palm. Some are simple globules or spheres, con- 

 sisting of .a single cell or little bag of tissue filled with a 

 colouring matter ; some are mere strings of such cells co- 

 hering by the ends ; others, a little more complex, exhibit 

 the appearance of branched threads ; in others, again, the 

 branches and stems are compound, consisting of several 

 such threads joined together ; and, in others, the tissue 



