CHAPTER II. THE THALLOPHYTA. 271 



groups at least as distinct from each other as the Pteridophyta 

 are from the Phanerogamia. But this confessedly imperfect 

 classification may still serve as a scaffolding with which to build 

 the more perfect structure. 



It is beyond the scope of this work to give more than a gene- 

 ral view of some of the principal types of plants under each 

 Series. 



CHAPTER II. 



SERIES I.— THE THALLOPHYTA. 

 Characteristics of the Series — Classes of Thallophyta. 



The name Thallophyta literally means " thallus-plants," and 

 it alludes to the fact that in this Series there is no clear differen- 

 tiation of the plant-body into root, stem and leaf. It includes a 

 vast number of forms differing widely from each other in struc- 

 ture, appearance and habit, some unicellular and the very 

 simplest and smallest of organisms known, others comparatively 

 complex and of large size. Not even the highest, however, ever 

 possess true roots, though some are provided with root-like, but 

 simpler organs, called rhizoids, which serve mainly for anchorage 

 or as holdfasts. A few of the higher forms show some differen- 

 tiation of stem and leaf, but in no case is this distinction as sharp 

 and clear as we find it in most plants belonging to the higher 

 groups, and in the great majority of cases it is entirely wanting. 

 While also the internal structure in some of the highest forms 

 attains a considerable complexity, there is never a clear differen- 

 tiation into epidermal, fundamental and fibro-vascular systems 

 of tissue, such as we find in Ferns and Flowering-plants. 



Between the highest and lowest forms of the Series, there are 

 various gradations of structure, and these are not always along 

 the same line of development. Among unicellular forms every 

 gradation may be seen between the simplest possible cells and 

 those of the highest degree of complexity, and among multicellu- 

 lar forms, there are those in which the cells are united in the 

 simplest possible way, namely, in a linear series to form fila- 

 ments, and the cells have so little dependence upon each other 



