Botanical Exhibits at the Royal Society. 
1 39 
ramentum-like appendages. It will be remembered that Botryo- 
pteris forensis of the French Permo-carboniferous isalso characterised 
by an abundance of somewhat similar dermal appendages. 
Colonisation ry Blue-Green Alg^e in the Tropics. 
Dr. Fritsch had arranged a small exhibit illustrating the 
colonisation of bare surfaces by sub-aerial Algae (Cyanophyceae) in 
the Tropics. In the moist lowlands of Ceylon most objects tend to 
be clothed with a dense growth of this group of Algae, and similar 
features are to be observed in any moist hothouse kept at a high 
temperature (e.g., the Nepenthes or Aroid houses at Kew). A series 
of rocks, and photographs of rocks, from the Nepenthes-house were 
shown in illustration of progressive stages in colonisation; and the 
exhibit was also illustrated by a few specimens from Ceylon and by 
microscopic preparations. The first blue-green forms to obtain a 
foothold are such as grow firmly attached to the substratum; 
according to the amount of rainfall and the temperature, this 
growth is either more or less dry and encrusting, or slimy and more or 
less gelatinous. Such adhesive growth gradually forms a compact 
investment, which prepares the way for algal forms exhibiting a 
higher type of growth ; adhesive growth rarely attains any great 
thickness, probably owing to the difficulties of respiration within 
the mass. Tangled growth, which generally colonises the surface 
of the adhesive forms at an early stage, is much more suitable from 
the respiratory point of view, and may form a thick stratum, serving 
as a basis for the growth of vegetation of a higher type (e.g., small 
Bryophytes or Ferns). Wherever there is plenty of moisture in 
the air, however, the filaments of a tangle tend to grow out vertically 
from the substratum and to give rise to a kind of tufted growth ; 
this is probably a response to a hydrotropic stimulus. Where such 
tufted growth is overshadowed it often assumes a well-marked 
stratified arrangement, often well seen on tree-trunks in the Tropics. 
When tangled or tufted growth first becomes colonised by small 
Bryophytes there is often keen competition between the two ; the 
Alga twines round the Bryophyte, thus raising its filaments into the 
air, whilst the latter grows on in front so as to escape the invading 
Alga. In this way a thick carpet of algal growth is gradually 
formed. The wealth of vegetation on every conceivable object in 
the moist Tropics is primarily due to the agency of the Cyanophyceae. 
