6 A TEXT-BOOK OF BOTANY. 



ductive organs which are analogous, even if they are not exactly 

 similar, to those of the flowering plants. Consequently the former 

 classification is no longer applicable, and the following arrange- 

 ment is now generally adopted : 



Thallophytes ....... 



( Fungi 



Archegoniates ...... ( Bryophytes 



( Pteridophytes. 



Spermophytes ...... (Gymnosperms 



[ Angiosperms 



In our study of these groups we shall see that in passing 

 from the Thallophytes through the various groups to the Angio- 

 sperms we pass from very simple forms to those which are quite 

 complex. 



THALLOPHYTES. 



General Characteristics. This group comprises those plants 

 which are simplest in form and structure. They are supposed 

 also to represent more or less primitive types. In these the plant 

 body does not show a differentiation into root, stem, and leaf, 

 as in the higher plants, and is termed a thallus, the word thallus 

 meaning a " mass " of cells. The cells making up a thallus are 

 all alike and are not differentiated for special functions. How- 

 ever, it must not be thought that every Thallophyte is charac- 

 terized in this way. Many of the Thallophytes have cells or 

 groups of cells which become specialized, i.e., set apart for a 

 particular function, as for example the reproductive cells. We 

 see, therefore, that the word Thallophyte is a general term and 

 is applied to many plants which are not thallus-bearing, but 

 which are really closely related to the simpler forms to which 

 the word Thallophyte is strictly applicable. When made up of 

 a mass of cells they may branch in various ways, but the essential 

 structure remains more or less uniform throughout. 



The Thallophytes vary in size and general appearance from 

 minute, unicellular organisms to those which are filamentous 

 and delicately branched, and even becoming leaf-like structures, 

 attaining a length in some of the marine algae of a thousand feet 



