DEVELOPMENT OF PLANTS 



261 



ers, are very hygroscopic; they coil and uncoil with the least 

 change in the humidity of the air and thus doubtless assist in the 

 gradual exposure of the spores to the air currents. The spores 

 germinate as in the lower liverworts. Thus 

 we see that the sporophyte, consisting of a 

 foot, stem and capsule, is more complex and 

 larger than in Ricciocarpus. This is doubt- 

 less due to the better nourishment which it 

 receives as a result of the development of a 

 more efficient absorbing organ, the foot. 



105. Order b. Jungermaniales or Leafy 

 Hepatics. By far the larger number of 

 hepatics belong to this order. They are espe- 

 cially abundant in moist tropical countries, 

 where they often cover the stems and leaves 

 with a rich vegetation, and with us they are of 

 common occurrence on dripping rocks and in 

 deep woods on the moist bark of trees and de- 

 caying logs or damp earth. In the lower form 

 the thallus is very simple and delicate (Fig. 

 196). In other genera there are indications of 

 a lobing in the thallus which becomes more 

 pronounced in some forms and leads by grad- 

 ual gradations to genera in which the lobes 

 appear as distinct leafy organs arranged in two 

 rows upon a stem-like axis (Fig. 197). Us- 

 ually a third row of rudimentary leaves, asso- 

 ciated 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 re- 

 main practically unchanged in all forms. Even 

 in the leafy genera, which comprise the ma- 

 jority of forms in this order, there is little evi- 

 dence of such a differentiation of the tissues 

 as characterize the Marchantiales. Without doubt, the simple 

 thallose Jungermaniales are the most primitive forms of the 



FIG. 196. One 

 of simpler thal- 

 loid Jungermani- 

 ales, Pallavicinia, 

 showing the rhi- 

 zoidal growth on 

 the under surface 

 of the thallus and 

 a mature sporo- 

 phyte arising from 

 a cup-like perianth 

 which is surround- 

 ed at its base with 

 involucrate leaves. 

 H. O. Hanson. 



