290 STRUCTURE OF LEAFY HEPATICS 



axis (Fig. 197). Usually a third row of rudimentary leaves, 

 associated with rhizoids, appears upon the under side of the thal- 

 lus (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 com- 

 prise the majority of forms in this order, there is little evidence 

 of such a differentiation of the tissues as characterize the' Mar- 

 chantiales. 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 Marchan- 

 tiales 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, consisting 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 con- 

 taining water (Fig. 197, B). This peculiarity 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 confused. In fact, 

 they are often called scale mosses, owing to the moss-like ap- 

 pearance 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 

 than 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 



