4 Introduction 



to the slowest river. They are most abundant, however, in still 

 waters, occurring in quantity in pools, ditches and lakes. 



They occur either floating at the surface, being simply immersed 

 in the water, or attached to submerged stones, or to larger aquatic 

 plants as epiphytes, etc. The larger Algse are often conspicuous 

 as green slimy masses forming a surface coat to ponds, ditches, etc., 

 or attached as large green masses to the rocks and stones of rivers. 

 Rocks over which the water is constantly dripping possess at times 

 quite a characteristic Alga-flora, and many of the more uncommon 

 Alga? are found in such situations. If such rocks present vertical 

 wet faces, they are often covered with thick leathery patches, or 

 gelatinous masses, which exhibit a great variety of colour. This 

 material is always of interest and usually consists of a mixture of 

 plants belonging to the blue-green Alga? or Myxophycea?. 



Some Algae have acquired a symbiotic relationship with other 

 plants, and even with animals. One species of Anabcena lives 

 symbiotically with the aquatic Lycopod Azolla, and another with 

 the Hepatic Blasia, and some Alga? belonging to the genus 

 Chlorella are connected symbiotically with such animals as 

 Hydra vlridis and certain small species of Amoeba, Paramecium, 

 Ophrydiwn, etc. Again, there is the Lichen, which is a compound 

 organism consisting of a Fungus associated symbiotically with 

 Alga? of the genera Plearococcus, Trentepohlia, Cephaleuros, Nostoc, 

 Stigonema, Scytonema, etc. The genus Foreliella has a symbiotic 

 relationship to the freshwater mussel (Anodonta) and several other 

 Algse are similarly related to sponges. 



Many of the most beautiful Alga? are exceedingly minute and 

 occur in quantity in situations which are not at first obvious, and 

 which are only found by experience. They occur embedded in a 

 thin mucus surrounding the stems and leaves of submerged plants, 

 such as Utricularia, Myriophyllum, Nymphcea, Nnphar, Potcnno- 

 yeton, Scirpus fluitans, Isoetes, etc. Many of the submerged mosses, 

 such as Sphagnum contort um, S. plumosum, Amblystegium scorpi- 

 oides, A. falcatum, A. exannidatum, A. glaitcu'in, Fontinalis anti- 

 pyretica, Jungermannia inflata, Nardia emarginata, etc., are often 

 richly covered with these minute Alga?. 



Numerous minute Alga? occur along with an abundance of the 

 Peridiniea? and Copepoda scattered through the surface waters of 

 lakes, rivers, and other large bodies of fresh water, and constitute 

 a large proportion of the Freshwater Plankton. The animal and 



