Chapter I 



CLASSIFICATION 



The general student is probably famihar with the seaweeds of the 

 sea-shore and may even have observed green skeins in stagnant 

 fresh-water ponds and pools. These plants, which are among the 

 simplest in the plant kingdom, belong to the group known as algae. 



However, there are many more representatives of this group than 

 the seaweeds, and it is the purpose of this book to give some ac- 

 count of them. The first step is to consider how they are classified 

 and the criteria upon which the classification is based. 



In the older classifications the algae proper were divided simply 

 into four principal groups, Chlorophyceae or green algae, Myxo- 

 phyceae (Cyanophyceae) or blue-green algae, Phaeophyceae or 

 brown algae and Rhodophyceae or red algae. However, now that 

 more is known about the Hfe history of many simpler organisms, 

 which used not to be regarded as true algae, it has been reaUzed 

 that the old classification must be extended and the number of 

 groups increased. This is because it has become evident that simple 

 unicellular organisms, formerly often placed with flagellates, must 

 properly be regarded as algae, even though of a very primitive 

 kind. The reason for this change of view was that botanists began 

 to reaUze, first in the case of the yellow-green algae (Xantho- 

 phyceae) and later with other primitive groups, that there were 

 analogues with the Chlorophyceae (cf. p. 324), and also that there 

 were a few additional 'algal' types even as defined according to the 

 classical concept. In the present work the Euglenoids (Euglena) are 

 regarded as true flagellates and not as algae even in the modern 

 more extended usage of that word. 



The principal criteria upon which the primary classification of 

 the algae are based rest upon differences in pigmentation and other 

 biochemical characteristics. Whilst there are one or two exceptions 

 to the general basis of differentiation, these are so few (and even 

 then somewhat uncertain) that they can be neglected. A study of 

 Table i, which summarizes the differences between the respective 

 algal classes, indicates that there are also resemblances between 



