THALLOPHYTA 355 



It must not be assumed that all the organisms thus grouped under 

 the common head of Thallophyta are necessarily akin to one another. 

 They are found to be naturally separable into distinct groups or 

 phyla. The plants belonging to the several phyla may be so arranged 

 as to show progress from simpler to more complex forms. Such 

 sequences probably represent with some degree of accuracy Lines 

 of Descent. Commonly the simpler terms of these distinct phyla are 

 more alike than those which are more advanced. Thus the Lines of 

 Descent are divergent, and the Thallophytes would therefore appear 

 to represent a brush of phyletic lines radiating outwards from some 

 simpler source. Though there is at present no trustworthy evidence 

 that any of the Thallophyta now seen living have themselves achieved 

 that highest development seen in the Land-Vegetation, many have 

 advanced far in their evolution. In mere size the Brown Seaweeds 

 include the gigantic Tangles, which are among the largest of living 

 organisms. In complexity of propagative method no group of 

 Plants shows more intricacy than the Red Seaweeds. In physio- 

 logical resource the Fungi are the most diverse. But each of these 

 includes simple types, which link up more easily with other classes of 

 organisms than do the extremes. Each phylum appears to have 

 worked out its own divergent line of advance independently of the 

 rest. Some degree of parallelism in the progressions may then be 

 anticipated, and is actually found to exist. 



The readiest basis of distinction of these natural groups of Thallo- 

 phyta is by colour. The most important cleavage is according 

 to the presence or absence of chlorophyll, or of some of its deriva- 

 tives. This separates the Fungi, which have no chlorophyll or 

 kindred colouring matters, from the Algae, which have. The Algae 

 again fall into distinct groups on the basis of colour-difference. Those 

 which have full green chlorophyll, such as is seen in Land-Vegetation, 

 are designated the Chlorophyceae. Others are characterised by 

 their olive green or brown colour, which is due to a mixture of pigments 

 of which chlorophyll is one. It is characteristic of the Brown Tangles, 

 or Phaeophyceae. A third series have a prevalent red colour also 

 due to a mixture of pigments, which again includes chlorophyll, and 

 they are called the Rhodophyceae, or Red Seaweeds. These colour- 

 distinctions are not absolutely constant but, together with other 

 characteristics, such as the nature of the flagella, or organs of propul- 

 sion of the motile stages, and the forms in which storage materials 

 occur in the cells, they afford a true indication of the several distinct 

 groups. 



