DEVELOPMENT OF PLANTS 277 



the Hepaticae. The cause of the differentiation of the tissues 

 in the two groups is doubtless explained by their relation to 

 moisture. The leafy hepatics are easily wetted and the entire 

 plant body can readily absorb moisture. Therefore the tissues 

 remain delicate and fewer rhizoids are required to absorb the 

 moisture. The Marchantiales are not wettable, and as a conse- 

 quence they must develop wick-like strands of rhizoids as well as 

 the elaborate storage tissues of the thallus. Both groups, and 

 indeed all hepatics, must remain prostrate, because they are 

 dependent upon surface water and have not as yet developed 

 an absorbing and conducting apparatus that will permit of any 

 other position. 



Reproductive Features of the Jungermaniales. — 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, termed 

 the perianth (Fig. 197, C), assumes very characteristic forms 

 in the difTerent genera and is generally associated with modified 

 leaves, the involucre. The gametospore develops after the man- 

 ner noted in Marchantia, forming a capsule with elaters, but 

 its stalk or seta reaches much larger dimensions, owing doubt- 

 less to the well-developed foot which penetrates the thallus and 

 absorbs foods, thus making possible a larger sporophyte (Fig. 

 197, E). When the spores have been matured the seta rapidly 

 elongates to several times its original length, rupturing the arche- 

 gonium and lifting the capsule high in the air (Figs. 196; 197, A). 

 The capsule usually breaks open into four valves which are hygro- 

 scopic, closing over the spores in damp weather and opening 

 in dry weather to expose them to the wind. These spores germi- 

 nate and begin the life history of a new gametophyte. As in 

 some of the Marchantiales a filamentous alga-like growth is 



