IV. THE ANTIIOCEROTES 



157 



capsule is unlimited. All that is needed to make the sporo- 

 phyte entirely independent is a root connecting- it with the 

 earth. 



The Inter-rclationships of the Ilcpaticcc 



From a review of the preceding account of the Liverworts, 

 it will be apparent that these plants, especially the thai lose 

 forms, constitute a very ill-defined group of organisms, f;ne set 

 of forms merging into another by almost insensible gradations, 

 and this is not only true among themselves, but applies also 

 to some extent to their connection with the M(jsses and 

 Pteridophytes. The fact that the degree of development of 

 gametophyte and sporophyte does not always correspond makes 

 it very difficult to determine which forms are to be regarded as 

 the most primitive. Thus while Riccia is unquestionably the 

 simplest as regards the sporophyte, the gametophyte is very 

 much more specialised than that of Aneura or Sphccrocarpus. 

 The latter is, perhaps, on the whole the simplest form we know, 

 and we can easily see how from similar forms all of the other 

 groups may have developed. The frequent recurrence of the 

 two-sided apical cell, either as a temporary or permanent con- 

 dition in so many forms, makes it probable that the primitive 

 form had this type of apical cell. From this hypothetical form, 

 in which the thallus w-as either a single layer of cells or with an 

 imperfect midrib like Sphcerocarpiis, three lines of development 

 may be assumed to have arisen. In one of these the differenti- 

 ation was mainly in the tissues of the gametophyte, and the 

 sporophyte remained comparatively simple, although showing 

 an advance in the more specialised forms. The evolution of 

 this type is illustrated in the germinating spores of the Mar- 

 chantiacea^, w^here there is a transition from the simple thallus 

 with its single apical cell and smooth rhizoids to the complex 

 thallus of the mature gametophyte. In its earlier phases it re- 

 sembles closely the condition which is permanent in the simpler 

 anacrogynous Jungermanniacese, and it seems more probable 

 that forms like these are primitive than that they have been de- 

 rived by a reduction of the tissues from the more specialised 

 thallus of the Marchantiacese. Sphcerocarpiis, showing as it 

 does points of affinity wuth both the lower Marchantiales and 

 the anacrogynous Jungermanniales, probably represents more 

 nearly than any other known form this hypothetical type. Its 



