276 FORM OF HEPATICS 



mentary leaves, associated with numerous rhizoids, appears 

 upon the under side of the thallus (Fig. 197, B). Although the 

 Jungermaniales tend to become highly modified into a leafy 

 body the tissues remain practically unchanged in all forms. 

 Even in the leafy genera, which comprise the majority of forms in 

 this order, there is little evidence of such a differentiation of the 

 tissues as characterize the Marchantiales. Without doubt, the 

 simple thallose Jungermaniales are the most primitive forms of 

 the hepatics and probably stand nearer the ancestral type than 

 any other form. The Marchantiales represent a line in which 

 the tissues of the thallus have attained a considerable degree of 

 differentiation, but in the present order, the evolution led to a 

 marked change of form with but slight modification of the tissues. 

 This is apparent in the highest forms where the leaves, consist- 

 ing of little more than a single layer of chlorophyll-bearing 

 cells, are arranged in two rather oblique rows upon delicate 

 stems. The under surfaces of these leaves are generally lobed 

 and often form sacs containing water (Fig. 197, B). This pe- 

 culiarity of the leaves, together with the rudimentary third row 

 of leaves that are associated with the rhizoids, makes a sharp 

 contrast between the upper and lower surfaces and serves to 

 distinguish them from the mosses, with which they are often con- 

 fused. In fact, they are often called scale mosses, owing to 

 the moss-like appearance of their dorsal surface and the close 

 contact formed with bark or other surfaces over which they 

 creep. This habit of pressing the leaves against the substratum 

 tends to retain the moisture and also enables the leaves to absorb 

 it directly through their delicate cell walls. The overlapping 

 of the leaves in many genera, as well as their lobing, would 

 serve the same purpose and enable the plants to live under drier 

 conditions that would otherwise be possible. Doubtless, these 

 departures have been of great advantage to the plants and in 

 part account for the common occurrence of these leafy forms. 

 It should be stated that the leafy hepatics are regarded as forms 

 that have been evolved from the simple thallose forms in quite 

 recent geological times and owing to their better adaptation to 

 present conditions the}' have become the most numerous of all 



