DEVELOPMENT OF PLANTS 263 



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 surface of these leaves are generally lobed 

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

 peculiarity of the leaves, together with the rudimentary third 

 row of leaves that are associated with the rhizoids, make 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 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 conditions they have become the most numerous of all 

 the Hepaticae. 



Reproductk'c Features of the Juugenuaiiiales. — Asexual re- 

 production is of the same vegetative type as has been seen 

 in the preceding groups. The archegonia and antheridia are 

 also of essentially the same character as seen in Marchantiales. 

 They are borne upon the dorsal surface of the thallus or upon 

 more or less modified branches, the archegonia often arising 

 upon the apex of the branch and the antheridia appearing as 

 rather spherical bodies in the axis of the leaves (Fig. 197, 

 C, D). The archegonia are developed in rather conspicuous 

 cup-like or leafy outgrowths of the thallus. This structure, 



