Phytogeography of the Southwestern Pacific 199 



it diminishes progressively with increasing heterotrophic nutri- 

 tion and in darkness is only about one-half that observed when 

 full photosynthesis is maintained. As Bristol points out, there 

 are likely to be considerable differences between different Algae 

 in this respect^ and, indeed, it is known that some Green Algae 

 prosper better when organic nutriment is available than when 

 it is not. 



It is therefore credible that subterranean Algae will find a 

 means of sustenance and even of active growth when buried in 

 the darkness of the soil, but there are two considerations that 

 must be borne in mind in this connection (cf. also Petersen,^ 

 p. 16). First, the Algae will be competing for organic nourish- 

 ment with the other living organisms of the soil and it seems 

 unlikely that this competition usually will be a successful one. 

 Second, there is no evidence so far that the kinds of organic sub- 

 stances that have proved efficacious in the artificial cultivation of 

 soil Algae are available in the soil, and there is therefore no direct 

 proof that an alga buried in the soil in darkness will find the 

 means of sustenance. This problem has recently been approached 

 by Petersen,"^ who has endeavored to ascertain whether two soil- 

 Algae {Pleurochloris magna and Nitzschia Ktitzingiana f. ter- 

 restris), inoculated into sterilized soil, exhibited any capacity for 

 growth in the dark. His tentative conclusions are in the negative 

 and, although the process of sterilization may conceivably have 

 induced alterations in the nature of the organic material avail- 

 able, the experiment certainly affords some evidence that these 

 two soil forms do not find suitable material for sustenance in the 

 soil in the absence of light. Esmarch,^^ p. 259, also failed to obtain 

 any evidence of growth in Blue-green Algae buried in the soil. 

 These investigations show that, until definite proof has been 

 obtained for a number of the regularly occurring soil Algae, 



