DEVELOPMENT OF PLANTS 291 



conditions they have become the most numerous of all the 

 Hepaticae. The cause of the differentiation of the tissues in 

 the two groups is doubtless explained by their relation to mois- 

 ture. 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 consequence they 

 must develop wick-like strands of rhizoids as well as the elabo- 

 rate 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 w r ill permit of any other position. 

 Reproductive Features of the Jungermaniales. Asexual repro- 

 duction 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 different 

 genera and is generally associated with modified leaves, the 

 involucre. By these devices the archegonia gain the same 

 protection against drying out as was secured to them by the 

 cavities in which they were developed in the preceding group. 

 The gametospore develops after the manner noted in Marchantia, 

 forming a capsule with elaters, but its stalk or seta reaches much 

 larger dimensions, owing doubtless to the well-developed foot 

 (Fig. 197. ). The majority of the genera also are characterized 

 by having many of the cells of the capsule sterilized so that 

 the number of spore-producing cells becomes further reduced. 

 When the spores have been matured the seta rapidly elongates 

 to several times its original length, rupturing the archegonium 

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

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



