210 



not distinctly visible in the dried material; they are therefore in many cases not 

 shown in the illustrations, or if so, only in small numbers, though as a matter of 

 fact, they are always present, or have at any rate been so. I mention this point, 

 as Mme Lemoink states that the cells are in some cases connected by open chan- 

 nels, (Struct, p. 35) and that in other instances, neither pits nor channels were found 

 (1. c. p. 37). As iUustrations of the latter, the writer in question cites Lithothamnion 

 IcEve and L. norveyicum; I can here refer to my figs. 129 and 143, where the pits 

 are shown. 



Pits between cells belonging to different filaments are found in the Danish 

 species only within the genus Lithophyllum, where the cells in the perithallium form 

 transverse layers, in which they lie at equal height, and are then connected by 

 pits with all the cells in the same layer, with which they are in contact. This has, 

 it is true, been known before, but the importance of the fact as a systematic 

 character has not been sufficiently emphasized. The character in question would 

 in particular seem to afford an excellent means of distinguishing between the genera 

 Lithophyllum and Melohesia, which otherwise closely resemble each other. Unfortu- 

 nately, I have not been able to ascertain how these pits arise; they are formed at 

 an early stage, and I must presume that they originate in a similar manner to the 

 secondary pits in the Rhodomelacece etc., though 1 have not been able to demon- 

 strate the co-operation of nuclei in the process, probably owing to insufficient fixing 

 and staining of the material. 



In all other genera (where, as we have seen, no such transverse pits are found) 

 the cells possess another means of entering into connection with cells in other 

 filaments, viz. by forming an open communication between them, the separating 

 wall being partially dissolved. These fusions, which were first described by Ro- 

 SANOFF, are of common occurrence in the Danish species of the family which do not 

 belong to the genus Lithophyllum 



Where the cells lie densely packed and the walls are thin, the fusions make 

 themselves apparent merely by the fact that the longitudinal walls are partially 

 dissolved {Lith. Lenormandi fig. 133 D); where the cell walls are thicker, on the 

 other hand, a distinct connecting channel of varying length is seen. These appear 

 both in the hypothallium and in the perithallium, and may very often take place 

 between more than two cells. They are particularly easy to distinguish in the basal 

 layer of Melobesia and in the central tissue of the upright, branched Lithothamnia. 

 In the latter, they often form characteristically curving partially branched bodies, 

 which may embrace almost all the cells in the central tissue (fig. 139). in the peri- 

 thallium also, however, of the mentioned Lithothamnia, they may be extraordinarily 

 frequent (Lith. calcarenm, fig. 144, etc.). Fr. Schmitz, who has investigated these fusions 

 with regard lo the behaviour of the nuclei, found in 1880 (Untersuch. iiber die 

 Zellkerne der Thallophyten. Sitzungsber. der niederrhein. Gesellsch. f. Natur- u. 



Of the species mentioned below, they appear to be lacking only in Choreonema Thnreti, where 

 tiie vegetation organs are highly reduced (cf. Minder 1. c.) and in M. ininutula (fig. 172). 



