376 THE ALGAE 



important biological significance, but at present much more experi- 

 mental data is required. It may well be that the biological character 

 of a soil will prove to be of more importance than its chemical or 

 physical character. 



Subterranean Algae 



There are great fluctuations in the numbers of the different 

 species that compose the flora, but there are no species in the lower 

 layers of the soil which do not also occur in the surface layers. 

 Dilution cultures, together with the counting of samples, have 

 shown that the algal flora is mainly confined to the top twelve inches 

 of soil with a maximum abundance at about 3-6 in. below the 

 surface. With increasing depth the number of algae decreases 

 regularly, the maximum depth at which they have been recorded 

 being two metres; there is, however, really no conclusive evidence 

 which shows that algae can grow in the deeper layers where there 

 is no Ught, and it is very probable that they are only present in 

 these layers in a resting phase. The number of reproductive bodies 

 in the surface layers reaches a maximum in spring, but in the lower 

 levels it remains constant throughout the year. In Denmark the 

 quaHty of the soil is apparently decisive in determining the luxuri- 

 ance of the flora irrespective of whether the ground has been dis- 

 turbed or not. In Greenland soil algae have been found down to a 

 depth of 40 cm., and their presence there can only satisfactorily be 

 explained by the action of water trickling down the cracks because 

 burrowing animals are absent. A study of soils from all over the 

 world has emphasized the existence of a widely distributed algal 

 formation in cultivated soils. This flora consists of about twenty 

 species of diatoms, twenty-four of Myxophyceae and twenty species 

 of green algae, among which Hantzschia amphroxys, Trochiscia 

 aspera, Chlorococcum humicolum, Bumilleria exilis and Ulothrix sub- 

 tilis var. variabilis are the most frequent. 



Lund (1945) studied sixty-six different soils in Great Britain and 

 found that, except on acid soils, the Chlorophycean component 

 was the most numerous, but it was followed closely by the Bac- 

 cilariophyceae with the Myxophycean element third. In the case of 

 cultivated soils, however, the Myxophycean element became much 

 more important. Very little work has so far been carried out on the 

 soils of tropical regions. A study of the soils of savanna, steppe and 



