1909-1910-] -^ ^S^ <^^^^ their Study. 237 



turn green, as can be easily observed on our coasts when 

 Algse are thrown up by the tides. 



Among the types of fresh-water Algae we find either single 

 cells or a colony of siagie cells arranged in a great variety 

 of forms ; or again, the cells are situated in a single row, as 

 we find among the filamentous Algae. With few exceptions, 

 all these cells are similar in shape as well as in function, and 

 we can therefore study here the primitive plant cell, in which 

 a division of labour, such as we find in the higher plants, has 

 not yet been evolved. As regards their reproduction, nowhere 

 in the plant world do we find such a variety of methods as 

 among the Algse, and even a simple enumeration of these 

 would take too long. [These were shown later on, with the 

 help of the lantern.] 



The algologists have a great advantage over botanists who 

 devote their attention to other groups of plants, in so far as 

 AlgfB can be found practically anywhere and at any time of 

 the year. It might perhaps be easier to eniimerate those 

 places where Algae cannot be found than tell where one 

 should look for them. Some practice, however, is required, 

 and the trained eye will see these minute organisms when 

 others will pass them unnoticed. Not only is running or 

 stagnant water the home of Algae, but we can find them in 

 great abundance in nearly every place where a moist surface 

 exists, such as damp walls, and especially rock faces over 

 which water is running. Although of microscopical size 

 individually, their presence can be detected by the green, 

 brown, purple, and even black patches which they form when 

 growing in great numbers, as is usually the case. Every one 

 has noticed the green film found on nearly every tree, wall, 

 or paling during all seasons : this is due in most cases to a 

 single -celled Alga known as Pleurococcus. On walls and 

 between paving -stones a light -green covering, which turns 

 to dark green in wet weather, is due to a filamentous Alga 

 called Prasiola. It is perhaps surprising that of the two 

 Algae just mentioned and of many unicellular types very 

 little is known to us. Even to-day a good deal of uncertainty 

 exists as regards the life-history and systematic position of 

 these, and a disagreement, very misleading to the amateur, is 

 noticeable in the books published by authorities on this subject. 



