214 



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



Fig. 2 ( x 72). — Tufted Growth of Tolypothrix, colonised by Bryophytes. The Alga is using 

 the latter as a support by means of which it raises its filaments up into the air. The 

 free ends of these filaments are appearing at all points. Only the sheaths of the 

 individual filaments are shown. 



the expense of the Symploca or Moss. Such cases very clearly illustrate the 

 tendency of the filaments of a tangle to grow away at right angles to the 

 substratum, and the stimulus is here undoubtedly a hydrotropic one. The 

 ultimate effect of Moss, etc., being used as a support by the Alga on which it 

 first settles clown will be to more or less kill the former, and it can only save 

 itself by growing out more and more as it gets entwined and killed off 

 behind. In this way a thick layer of humus will gradually accumulate and 

 serve as a base for the development of higher plants (small ferns, etc.). 

 This is probably the way (after preliminary adhesive and tangled growth) in 

 which bare rock-surfaces become gradually converted into substrata suitable 

 for all kinds of growth. The subaerial Cyanophycese play a great part in the 

 progressive colonisation of bare surfaces in the tropics, and to their agency 

 the wealth of vegetation on every conceivable object is primarily due.* 



* This subject will receive fuller treatment elsewhere (see "The Role of Algal Growth 

 in the Colonisation of New Ground, etc.," in a forthcoming number of the ' Geographical 

 Journal') ; cf. also Treub, 'Ann. Jard. bot. Buitenzorg,' vol. 7, 1888, p. 213. 



