THE BRYOPHYTA: HEPATICAE, THE LIVERWORTS 391 



A Fungus mycelium is frequently, if not always, found in the cells of the 

 lower part of the thallus, especially near the midrib, but is not normally 

 found in the sporophyte. This Fungus is supposed to live symbiotically 

 with the Liverwort. A similar endophytic Fungus is found in many other 

 Hepaticae, but little is known about their biological relationships. 



Fig. 372. — Pellia epiphylla growing on a shady bank. Half natural size. 



Growth takes place by means of a single, large, apical cell, which lies 

 at the base of a little depression between two lobes of the thallus. It cuts 

 off segments both below and on its two sides. From the former the midrib 

 is built up, whilst from the lateral segments the two lobes are developed. 

 Since this lateral tissue grows more rapidly than the apex itself, it follows 

 that the apical cell will always remain in a slight depression, thus being 

 protected to some extent from damage. This growing apex is further 

 protected by the development of hairs which secrete mucilage, and this 

 prevents the apical cell from becoming desiccated. 



The thallus branches dichotomously, and the beginning of this branching 



