DIVISION V. 

 THALLOPHYTA. 



CHAPTER XXIII. 

 THALLOPHYTA. 



INTRODUCTORY. 



THE Seed-Plants and the Archegoniatae constitute by far the greater 

 part of the Flora of the Land. But when these are put on one side, 

 there still remains a residuum of organisms properly ranked as Plants, 

 very numerous, though often individually small and inconspicuous, 

 and many of them dwellers in water. They are collectively designated 

 the " THALLOPHYTA," or thalloid plants, since a general feature in them 

 is the absence of that differentiation of the shoot into axis and leaf 

 which is characteristic of the higher forms. It must not be assumed 

 that all the organisms thus grouped under a common designation are 

 necessarily akin to one another. They are found to be naturally 

 separable into distinct groups or phyla. The plants belonging to 

 these 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 the more advanced. Thus 

 the lines of descent are divergent, and the Thallophytes would appear 

 to represent a brush of phyletic lines radiating outwards from some 

 simpler source. 



Though none of the Thallophyta have themselves achieved that highest 

 development seen in 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 propa- 

 gative method no group of Plants shows more intricacy than the Ked Seaweeds. 



