Algal Flora of l he Tropics. 
239 
like Pleurococcus) ; but even here numerous Chroococcaceous colonies were 
intermingled, and threatened ultimately to completely crowd out the 
green forms. Occasional colonies of green forms in very subordinate 
amount are sometimes to be found amid the elements of the blue-green 
growth, but their number is so small that from a general point of view 
they are negligible. The great preponderance of blue-green forms is no 
doubt in part due to special favouring conditions (high temperature, 
humidity of the air), but in part also to the dominant factors in the 
tropics (intense light, frequency of and extreme degree of desiccation) 
being unsuitable to the growth of pure green forms. Light probably 
plays a great part in the exclusion of the latter, the successful blue-green 
element having a protective pigment in the form of the phycocyanin, which 
accompanies the chlorophyll in the cells. 
We will now examine the literature with reference to this point. The 
statements quoted on p. 238 illustrate the fact that such records of an 
abundant occurrence of subaerial Algae in the tropics as exist refer to 
Cyanophyceae. All the more extensive algal floras show a relatively very 
large blue-green element, although no one actually mentions the dominance 
of the latter group in the subaerial vegetation l . An analysis of the tropical 
floras from this point of view will be found on p. 244, and it is unnecessary 
to say more on this subject here. The following is a list of the more 
important records of the occurrence of green subaerial forms in the 
tropics : — 
1. Pleurococcus crenulatus, Hansg. — Schmidle, ‘97 e, p. 258 ( ( Am Holze der 
Regenrinnen, Savaii. Das getrocknete Zelllager ist gelblichgriin ’) ; Lagerheim, 
’ 90 , p. 6 (on branches of Ilex scopuloruvi). 
2. Pleurococcus Kiitzingii , West. — West, ’ 04 , p. 287 (‘Barbados. — Bay Estate; 
forming a yellow-green stratum about 1 mm. in thickness, with a species of 
Lyngbya ’). 
3. Pleurococcus miniatus (Kiitz.), Nageli. — ’Lagerheim, ’ 90 , p. 6 (‘ En las piedras 
humedas en el jardin botanico ’). 
4. Pleurococcus vulgaris , Menegh. — West, ’ 04 , p. 287 (‘Barbados. — Bay 
Estate’); De Wildeman, ’ 97 , p. 80 (Jard. bot. de Buitenzorg) ; West and West, 
’02 b, p. 200 (‘ On decomposing surface of gneiss, Matara’); Mobius, ’95, p. 175 
(‘ An einem Felsenabhang, Tubarao’) ; De Wildeman, ’00, p. 109 (‘ Sur les ecorces 
d’arbres, sur les murs, etc. — Jardin bot. de Buitenzorg’); Zeller, ’73, p. 184 (‘ Ran- 
goon, ad parietes hospitii “circuit house” dicti ’) ; Hieronymus, ’95, p. 22 (no habitat 
mentioned); Lagerheim, ’ 90 , p. 6 (tree-trunks); Bohlin, ’97, p. 12 (Paraguay). 
5. Protococcus botryoides (Kiitz.), Kirchn.— West and West, ’97 a, p. 239 
(‘ Loanda. A light-green cover, often remaining dry, on the inner part of wooden 
water reservoirs ’). 
1 Lemmermann (’ 05 , p. 609) comments on the absence of Hormidium and Pleurococcus from 
the subaerial algal flora of the Sandwich Islands. 
