OF ARTS AND SCIENCES. 61 



order and in the same directions as in Tuomcya and Batrachosper- 

 mum. But in Lemanea these lateral cells do not grow out into dis- 

 tinct and separate ramelli, but remain as members of a solid tissue 

 which finally develops into the tubular frond characteristic of Lema- 

 nea. At the tips, then, of the branches and branchlets, the frond of 

 Tuomeya is essentially like that of Batrachospermum, and unlike that 

 of Lemanea. But its adult structure is just the opposite. Batracho- 

 spermum always retains the whorls, distinct and evident, and never 

 compacted into a structure resembling a cortex. Tuomeya, on the 

 contrary, as we have seen, by the dense growth of the ramelli and 

 the intertwining of their branchlets produces at a comparatively early 

 period a hollow and cylindrical frond, very much resembling that of 

 a Lemanea. It lacks completely, however, the highly specialized 

 lateral tubes so chai'acteristic of that genus. 



The growth of the secondary filaments in Tuomeya agrees with 

 that of the secondary filaments in Lemanea. In Batrachospermum 

 the secondary " corticating " filaments frequently branch in many 

 species, and the branches resemble very closely those of the ramelli ; 

 but in Tuomeya and Lemanea they branch only at rare intervals, 

 and, becoming very much thickened, materially increase the rigidity 

 of the axis. The secondary filaments in Tuomeya, however, are not 

 so completely modified as those of Lemanea. They retain some 

 active vegetative functions, and have some special duties, as will be 

 seen later. 



In the younger portions of a frond of Tuomeya, the branches origi- 

 nate in a uniform manner, but in an irregular sequence. Here a 

 branch usually takes the place of a ramellus, or of some portion of 

 a ramellus. The first evidence we have that a branch is about to be 

 formed is the appearance, as one of the members of a whorl, of a short 

 filament, resembling the apical protruding filament of the tip of a 

 branch, such as is mentioned above. This short filament increases 

 in length, and soon has the protuberances along one side which are 

 to grow out into the ramelli. In the forming branches, the protuber- 

 ances are usually borne along the lower surface (cf. Fig. 5). In the 

 older portions, the branches originate in a somewhat different fashion. 

 A filament, resembling one of the secondary filaments, grows out in 

 more or less of a horizontal direction, and when about to protrude 

 beyond the surface of the frond begins to develop in the same fashion 

 as a young branch (cf. Fig. 11). Young branches may also take 

 their origin from the more distal cells of a ramellus. 



Whenever the tip of a branch is injured, and its apical growth con- 



