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



attaining greater thickness than an adhesive growth) before its surface became 

 colonised by other kinds of Algae (exhibiting tufted growth). In the few 

 cases in which a thick tangled stratum was observed, only the external portion 

 was generally found to be living, the inner part consisting largely of dead 

 sheaths. The tangled type of growth leads to the retention of a considerable 

 amount of air in the interspaces between the filaments, and this is probably 

 of advantage from a respiratory point of view in comparison to the adhesive 

 habit ; this suggestion has, however, first to be confirmed by experiment. On 

 the other hand, this mode of growth (like the adhesive one) is not well adapted 

 to the absorption of atmospheric moisture, since only few of the filaments 

 project freely into the air. Such growth is probably largely dependent for its 

 supply of moisture on the substratum on which it grows, and this is in 

 agreement with the fact that it prospers best on clay embankments or porous 

 rock-surfaces. As it gets thicker, it very often passes over into tufted growth 

 (see below and fig. 1, C) or becomes overgrown by the tufted growth of another 

 more successful form.* 



The tangled type of growth is met with in subaerial forms of Tolypothrix, 

 Hapalosiiohon, Scytonema, Stigonema, etc., and in nearly all cases the filaments 

 are provided with firm consistent sheaths. In most of these genera (mainly 

 in Stigonema minutum, Hass. ? and species of Tolypothrix) one occasionally 

 meets with tangles growing in protected situations, and exhibiting a 

 characteristic association with a film of air which gives their surface a bluish 

 grey or bluish colour and a silvery sheen, and not only occupies the inter- 

 spaces of the tangles, but covers their entire surface. The forms under 

 consideration seem to avoid direct moisture as much as possible, and grow in 

 situations which are more or less protected from the rain. The air is held so 

 firmly in the interspaces of the tangle that it has not been dislodged by the 

 preserving fluid, but it seems probable that the impact of falling rain-drops 

 would be able to displace it, and that this is the reason why these forms are only 

 found growing in sheltered situations. The species exhibiting this association 

 with air, however, possess another peculiarity in the form of an irregular 

 incrustation on the sheath, completely absent at some points, but forming an 

 opaque covering which obscures the enclosed trichomes at others. In some 



* It may be pointed out, however, that both from the point of view of respiration and of 

 absorption of moisture the various species and genera of Cyanophycea 1 may differ markedly 

 among one another ; thus one form may be much better enabled to withstand the draw- 

 backs of an adhesive or tangled growth than another. That, under certain circumstances, 

 a tangled growth may be laden with moisture is evidenced by the fact that in damp 

 protected localities I occasionally found aquatic Algse (e.g., species of XJlothrix) growing 

 on the surface of the tangle. These Algse probably obtain the necessary moisture by 

 absorption from the tangle below. 



