28 MOSSES AND FERNS chap. 



the latter, which thus comes to he 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 ^ and R. crystallma 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. crystallma, 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 in R. fluitans are small and do not 

 become separated into two, and in R. crystallma 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 ^ 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 corresponding 

 increase in the number of the marginal cells. The central 

 cells of the marginal group now begin to grow more vigorously 

 than the others and to project as a sort of lobe (Fig. 3, C, L), 

 and this lobe divides the initial cells into two groups lying 

 on either side of it. As soon as this is accomplished each 

 new group of initials continues to grow in the same manner as 

 the original group, and two new growing points are established, 

 each of which develops a separate branch. The growth of the 

 middle lobe is limited, and it remains sunk in the fork between 

 the two new branches. 



The thallus is attached to the substratum by root-hairs of 

 two kinds. The first are smooth-walled elongated cells, with 

 colourless contents, the others much like those of the higher 

 1 Leitgeb (7), vol. iv. p. 11. , - Fellner (i). 



