III. 

 Adaptation in Liverworts. 



IT is well known among bryologists that the liverworts, unlike their 

 relatives the mosses, are not stereotyped in form, but vary 

 enormously, and in many different directions ; they have in conse- 

 quence been aptly called " one of Nature's experimental grounds." 

 At the outset we have the two great groups of the foliose and thallose 

 — or, according to a more recent and nearly corresponding division, 

 the acrogynous and anacrogynous forms, with their strikingly different 

 habits of structure and growth, though they are well connected by 

 intermediate forms. Within tliese divisions, but especially in the 

 acrogynous, there is still an extraordinary variety of form to be seen, 

 and this group will be chiefly dealt with here. 



The most important variations are connected with the collection 

 and retention of moisture, as might be expected in plants presenting 

 a small surface, and developing unicellular root-hairs only, which can- 

 not penetrate the substratum to any considerable depth. Results of 

 investigation in this department have been recently published, which 

 show a remarkable power of complex adaptation in these lowly plants 

 (i, 2). 



The typical form of the acrogynous Jungermannieae is a thin, 

 usually more or less prostrate, stem, bearing three rows of leaves, two 

 lateral and one ventral. Those of the ventral row (amphigastria) are 

 usually very much smaller than the others, and are often absent 

 altogether; in the latter case the ventral segment of the three-sided 

 apical cell forms stem tissue only, while the lateral segments produce 

 each a row of leaves. The leaves are commonly bidentate or bilobed, 

 and in the simpler forms, as in the common Lophocolea bidentata, the 

 lobes lie in the same plane, so that there is no provision for retaining 

 moisture other than the capillary action among the leaves and stem. 

 Such forms commonly possess well-developed root hairs, some- 

 times in enormous numbers ; while in a few species, e.g., Lepidozia 

 reptans, there are whip-like branches bearing only rudimentary leaves, 

 but densely clothed with root hairs, which are apparently modified for 

 this purpose. 



Coming now to those forms which possess some kind of water- 

 collecting apparatus, we find, as Goebelhas shown, four chief classes, 



