222 SCIENCE PROGRESS 



yield the highest number of seedlings and to show the longest 

 germination period. Size of seed also appears to be a factor 

 influencing the percentage of germinating seeds, since this 

 latter is higher in the case of large seeds (e.g. Pinus ponderosa 

 and Pseudotsuga taxifolia) than in the case of small ones. 



The most complete account yet published dealing with the 

 ecology of terrestrial algae is that by J. B. Petersen {Mem. 

 de V Academie Roy. des Sciences et des Lettres de Danemark, 

 I. XII. 191 5). As a result of his observations he concludes 

 that diatoms are to be encountered on most types of bare 

 earth. Diatoms are, however, often absent from the soil of 

 woods and forests and the drier parts of heaths, especially 

 where there is a covering of decaying leaves. Such situations 

 are usually acid, and perhaps the poverty of diatoms is related 

 to this feature. The fact that on the wetter parts of heaths 

 several species occur might be due to the greater dilution of 

 the humic acids ; the author, however, expresses the opinion that 

 chemical reaction has little influence, although he finds that the 

 richest diatom flora is encountered on garden mould and arable 

 land where the acidity is usually relatively low. The most 

 interesting fact is that the aerial diatoms are not only distinct 

 from the aquatic, but are, without exception, of small size. 

 They are mostly members of the sub-family Raphidese, and the 

 author attaches importance to the fact that all can exhibit 

 movement, and so perhaps wander to the more humid situa- 

 tions during periods of drought. The aerial Chlorophyceae 

 appear to be distributed in relation to the character of the 

 surface, and chemical reaction. Especially characteristic 

 of acid soils are Zygogonium ericetorum (cf. Fritsch. Ann. Bot. 

 Jan. 1916), Mesotcenium violascens and Coccomyxa spp. On 

 alkaline soils Mesotcenium macrococcum and species of Hormi- 

 dium and Vaucheria. Other associations recognised are those 

 of dead wood, stony soils, tree trunks, etc. The epidendrous 

 forms are particularly abundant on the smaller branches, a 

 feature attributed to the diminished competition with lichens. 



The influence of the physical character of the substratum 

 is illustrated by the difference in the algal flora from the trunks 

 of various species of trees. The absence of algae from the 

 rain-tracks upon the trees is an interesting and surprising 

 feature. 



Genetics. — Miss Saunders has studied the inheritance of 



