II MUSCINEJE— HEPATIC^— MARCHANTIALES 27 



are often scarcely to be detected except close to the growing 

 point. 



In the case of Ricciocarpus natans (Leitgeb (7), iv., p. 29) 

 instead of a single scale being formed, each cell of the horizon- 

 tal row, which ordinarily gives rise to a single scale, grows 

 out independently, much as do the dorsal surface cells in the 

 other species, and the result is a horizontal series of narrow 

 scales, each one corresponding to a single cell of the original 

 row. These later are displaced by the subsequent growth of 

 the thallus, and their arrangement in transverse series can only 

 be seen in the younger parts. The very rapid increase in length 

 of the dorsal rows of cells as they recede from the growing 

 point soon causes them to overarch the latter, which thus comes 

 to lie in a deep groove ; indeed not infrequently the end cells of 

 the rows on opposite sides of the groove actually meet, so that 

 the groove becomes a closed tube. 



R. fluitans (Leitgeb (7), iv. p. 11) and R. crystallina differ 

 in some respects from the other forms. In these, owing to a 

 greater expansion of the tissues of the older parts of the thallus, 

 the air-spaces are very much enlarged. In the former they are 

 almost completely closed above, as the epidermal cells, by 

 repeated vertical divisions, keep pace with the growth of the 

 thallus and form a continuous epidermis, with only a small 

 central pore over each of the large air-chambers. In R. crys- 

 tallina, however, there is no such secondary growth of the 

 epidermal cells, and in consequence the cavities are completely 

 open above, so that the surface of the thallus presents a series 

 of wide depressions separated by thin lamellae. These two 

 species also show some difference as to the ventral scales. 

 Those of R. Huitans are small and do not become separated into 

 two, and in R. crystallina they are wanting entirely. 



Most of the Ricciacese multiply by special adventive shoots 

 that arise from the ventral surface of the midrib. These 

 become detached and form new individuals. According to 

 Fellner ( i ) the rhizoids develop at the apex a young plant in a 

 manner entirely similar to that by which the young plant arises 

 from the germ tube of the germinating spore. 



By far the commonest method of branching in most species 

 of Riccia is a true dichotomy. The first indication of this 

 process is a widening of the growing point and a correspond- 



