CH. XI] 



THE LIVERWORTS 



483 



FIG. 342.Antho- 

 natural 



The walls of the 



ing two horizontal ranks of very delicate 

 leaves (Fig. 341). These are scale-like, 

 one cell-layer thick, without midrib, and 

 very pleasing to see under the microscope. 

 In some epiphytic forms the lower lobes 

 of these leaves are developed to water- 

 holding sacs ; and many kinds have also 



a third row of smaller 



leaves along the under 



side of the stem. The 



antheridia occur among 



the leaves on side 



branches, but the arche- from the coiumeiia, 



gonia mostly terminate 



the main shoots. The 



sporogonium develops an elongated stalk 



and opens by four valves. Vegetative 



reproduction also occurs through various 



special bodies. 



ORDER 4- ANTHOCEROTALES : THE AN- 

 THOCEROS AND KIN. These inconspicuous 

 Liverworts form a small order, but are 

 highly interesting because their structure 

 suggests a possible mode of transition be- 

 tween Bryophytes and Pteridophytes. The 

 thallus is simple, and rests on the ground 

 like a Riccia (Fig. 342); but the 

 archegonia and antheridia are sunken 

 in the upper surface. The striking 

 feature is the sporogonium (Fig. 

 343), which differs from that of 

 othe r Liverworts in several respects. 

 It has a very slender elongated 

 form, and grows continuously from 

 the base; the spores are borne not 



shaded. 



