212 Dr. F. E. Fritsch. On the Subaerial and ' [Jan. 2, 



elongation of the filaments, which hang down in long tresses (e.g., T. arborum 

 (Ag.) De Willd.). 



The last type of growth presented by the subaerial Cyanophyceae is really 

 only a modification of the tufted one ; it may be spoken of as " stratified." It is 

 found in the same species as exhibit the tufted habit, although on the whole it is 

 of rare occurrence — a result, probably, of the absence of the special conditions 

 which lead to its development. It was mainly observed in species of TolypotTvrix 

 and Scytonema. The characteristic feature of this type of growth lies in the 

 fact that the closely-placed tufts are arranged in regular strata forming tiers 

 one above the other; each tier generally projects a little beyond the one 

 above it, and in this way a habit recalling that of some species of Hypnum* or 

 one of Schimper's " Etagenbaume " is attained. Such growth is mainly to be 

 found on vertical walls or tree-trunks, where the species of Alga concerned 

 often formed huge coherent sheets, which with some little trouble could be 

 detached in their entirety. Since this type of growth Occurs mainly in shaded 

 situations, it may be suggested that the projection of each tier beyond the 

 one above it enables the free growing ends of the tufts to obtain the maximum 

 amount of light ; in some cases these free ends were differently coloured to the 

 main inner mass, and these differences in coloration may be a result of the 

 illumination (see p. 206). It seems possible, therefore, that " stratified " 

 growth is the result of tufted growth on a vertical surface in a shaded 

 situation, and that stratification meets the requirements of illumination com- 

 bined with the largest possible extent of surface-development. But moisture 

 may also play some part in determining this type of growth, for, if one tier of 

 tufts is once established, others might gradually grow out below and elongate 

 sufficiently to catch up the drops of rain running off from the tier above ; in 

 this way tier upon tier might be formed. In this connection two points may 

 be noted : Firstly, the tufts of a tier in their entirety point downwards, so 

 that superfluous rain-water readily runs off one tier on to the next ; and, 

 secondly, " stratified " growth occurs on substrata which probably do not 

 furnish much water in themselves. Both illumination and moisture may 

 thus be concerned in the development of the stratified mode of growth, but I 

 am inclined to regard the former as more important. 



A few words may be added on the localities in which these different types 

 of growth seem to predominate, and on their inter-relation. In exposed 

 situations in the wet regions it is usual to find only one or other of the last 

 three types of growth (mainly the tufted one), so that ordinarily only such 



* E.g., Hypnum (Hylocomium) splendens. Cf. Goebel, ' Organographie d. Pflanzen,' Jena, 

 1898 — 1901, p. 56, fig. 27. Such growth is, of course, of quite a different nature to the case 

 we are considering, but there is a considerable superficial resemblance. 



