270 Algae. 



of the different types of waters in Ce^^lon: thus those found in the 

 rock-pools are, on the whole, quite different to those met with in 

 the clayey pools. The Cladophoraceae are mainly represented by 

 species of Pithophova, whilst Cladophora and Rhisoclonium appear 

 normally only to occur in well aerated habitats (e. g., wells, rivers, 

 artificial waters). The reason for the absence of these two genera is 

 probably to be found in their large, thickwalled cells, the walls 

 preventing an adequate respiration, alreadj^ seriously affected by 

 the small amount of dissolved oxygen in tropical waters; PithopJiora , 

 on the other hand, has thin walls, and may also benefit by the ease 

 with which it forms akinetes in times of desiccation. The genus 

 Vmicheria was only met with in the uplands; Botrydmm. was not 

 found. The scarcity of these forms may be related to their peculiar 

 type of assimilation, since the whole group of the Confervales, 

 which have a similar assimilatory process, are likewise very rare in 

 Ceylon. Diatoms are onl\^ met with in abundance in well aerated 

 waters; otherwise their number is very insignificant and they are not 

 uncommonly completel}^ absent. Apart from the species of Spivogyra 

 and PithopJwm the prevalent habit of the filamentous algal flora is 

 very narrow; forms exceeding 15 ,u in diameter are rare and many 

 of the Algae have considerably narrower filaments. This is due to 

 the increased difficulty of respiration in tropical waters; well aerated 

 waters show a much broader filamentous flora. The narrowness of 

 the filaments ist most striking in .the case of Oedogonium, species 

 of which are frequentl}^ quite an important component of the algal 

 Vegetation. All the above points receive some confirmation from 

 other tropical algal floras, although the data in these latter are 

 mostly so insufiicient as to make them of little use from these points 

 of view (see Annais of Botan5^ April 1904.) 



The luxuriance of subaerial algal growth in the tropics is a 

 result of a considerable amount of moisture, combined with a high 

 temperature ; a decrease in the amount of rainfall involves a marked 

 decrease in the extent of the algal covering. Where moisture and 

 high temperature combine, however, rocks, walls, tree-trunks, etc., 

 are all clothed with a thick carpet of Cyanophyceae. These exhibit 

 four main types of growth: the adhesive, tangled, tufted, and strati- 

 fied. Adhesive growth is the first to colonise new ground, and con- 

 sists of forms, which grow firmly attached to the substratum. A 

 thick growth of this kind is impossible, owing to the difficulties of 

 respiration within the mass; tangled growth, in which the filaments 

 grow loosely interwoven with one another, so as to form a tangle 

 with numerous interspaces filled by air. is better from this point of 

 view and, consequently, generally succeeds adhesive growth. In 

 moist localities there is a marked tendency for the filaments of a 

 tangle to grow out into tufts vertically away from the substratum; 

 this is probably the result of a hydrotropic Stimulus and gradually 

 gives rise to a thick layer of tufted growth. This seems to be the 

 habit best suited to the prevailing conditions. In shaded localities 

 with a considerable rainfall these tufts assume a definite stratified 

 arrangement, probably an adaptation to the conditions of Illumina- 

 tion. Both tangled and tufted growth form a suitable base for the 

 growth of small Bryophytes, which are often found in large num- 

 bers on these subaerial algal carpets. Then a struggle for supre- 

 macy commences between Alga and Bryophyte. The former begins 

 to twine itself round the stem and leaves of the latter, thus raising 

 its filaments into the air, while the Bryophyte grows on in front to 



