202 



BOTANY 



B 



CLASS I. HEPATIC^; 



The lowest of the Archegoniates are the Hepaticse, or Liverworts, 

 which are of importance, botanically, because they probably repre- 

 sent the forms from which all the higher types of green plants have 

 come. They are usually of small size, and most of them frequent 

 moist, shady places, although many species have adapted themselves 

 to dry localities. They reach their greatest development in the moist 

 mountain forests of the Tropics, where they occur in great numbers 

 upon the stems, or even the leaves, of many trees and shrubs. 



The Gametophyte 



The gametophyte in the Liverworts shows considerable range of 

 structure. The simplest forms have a thallus composed of nearly uni- 

 form cells, or with a midrib consisting of elongated cells (Fig. 172, B). 



The branching is most commonly di- 

 chotomous. The thallus is fastened to 

 the substratum by delicate unicellular 

 root-hairs. The growth of the thallus is 

 due to the divisions of a single apical cell 

 (Fig. 173, A, x). Most Liverworts, in 

 their earlier stages, conform to this type. 

 From this simple thallose structure, 

 specialization has developed in two 

 directions. In the Marchantiales the 

 thallose form has been retained, but 

 the uniform tissues of the simpler 

 type have been replaced by tissues 

 suited to special purposes. The green 

 cells occupy the dorsal part of the 

 thallus, and constitute a well-developed 

 assimilating apparatus, and the reproductive organs are often 

 restricted to special branches. 



The second line of development is seen- in the leafy Liverworts, 

 or Scale Mosses. The tissues in this type remain alike, but the 

 plant-body becomes a leafy axis, the assimilative function being 

 relegated to special outgrowths (leaves) (Fig. 172, F). These leafy 

 shoots sometimes arise as outgrowths of a thallose " Protonema," 

 like that found in the True Mosses. This protonema may be a flat 

 thallus (Lejeunia metzgeriopsis) (Fig. 180), or it may be filamentous 

 (Protocephalozia) . 



Reproduction. The gametophyte multiplies normally by branch- 

 ing, but in many Liverworts special buds or gemmae are developed. 

 In Aneura multifida, these are two-celled bodies, which are formed 

 Inside a mother-cell, and are discharged much like the zoospores of 



FIG. 163. Ricciocarpus natans. 

 A, floating form. B, terrestrial 

 form (X 2). 



