1907.] ' Fresh-water Algal Flora of Ceylon. 211 



These forms, and especially those exhibiting pure tufted growth (i.e., in which 

 there is little or no tangled base), readily absorb moisture from the substratum 

 by capillary attraction, whilst as long as the air is relatively dry (as compared 

 with the substratum), water will also be given off by evaporation from the 

 outer ends of the tufts, so that a constant circulation of water will take place. 

 On the other hand, when tufted growth occurs on a substratum which does 

 not furnish much moisture, the outer ends of the tufts will carry on the 

 reverse process, and absorb water- vapour directly from the air, provided the 

 latter is sufficiently moist. The tufted type of growth is thus liable to be 

 successful on all kinds of substrata provided the air contains the necessary 

 amount of moisture. These suggestions as to the biological value of tufted 

 growth seem to agree with the main facts of the distribution of the latter. 

 We should expect not to find tufted growth or to see it restricted to well- 

 protected situations in dry localities, in which both air and substratum furnish 

 little moisture ; and this is really the case, for at Anuradhapoora, Negombo, 

 etc., such subaerial growth as does occur is almost exclusively adhesive or 

 tangled.* On the other hand, the wetter the locality the more abundant does 

 this type of growth become ; even at Kew it only occurs in the very damp 

 hothouses (e.g., the Nepenthes-home). 



Atmospheric moisture thus seems to be the chief factor determining the 

 development of tufts, and there is good reason to look upon them as being 

 the result of a hydrotropic stimulus (cf. also below). The subaerial Cyano- 

 phycese under discussion, although they have left the aquatic habitat, are 

 still semi-aquatic in their requirements, and it is not surprising to find 

 moisture influencing them in various ways (i.e., general distribution and mode 

 of growth). In moist localities (i.e., where there is plenty of water- vapour in 

 the air) the advantage of the tufted as compared with the tangled type of 

 growth is obvious, especially in exposed situations where the surface layers 

 of the soil are very liable to be dried up by the heat of the sun's rays, so that 

 absorption of atmospheric moisture is imperative. In damp localities, probably, 

 all tangled growth sooner or later passes over into a tufted one or becomes 

 colonised by some other species with tufted growth, but in dry localities the 

 latter affords too ready a means of evaporation and, consequently, is unfavour- 

 able. Without experimental study the conditions of respiration in this form 

 of growth can hardly be compared with those in a tangle ; in all probability, 

 however, they are much the same in the two cases. 



Some of the species of Trentepohlia quite conform to the above-described 



method of growth (e.g., T. abietina, Hansg.), whilst others differ in the great 



* These observations were, of course, made during the dry season ; possibly during the 

 wet period tufted growth may be more commonly developed at these localities. 



