+---------------+-----------+------------------------------+ | | | Number of Species. | | | Number of +-----------+-----------+------+ | | Samples | Maximum | Average | | | Group. |Productive.|per Sample.|per Sample.|Total.| +---------------+-----------+-----------+-----------+------+ | | per cent. | | | | |Diatoms | 95·5 | 9 | 3·7 | 20 | |Blue-green algæ| 77·3 | 7 | 2·5 | 24 | |Green algæ | 100 | 7 | 4·3 | 20 | |Moss protonema | 100 | -- | -- | -- | +---------------+-----------+-----------+-----------+------+ | Total | -- | 20 | 10·5 | -- | +---------------+-----------+-----------+-----------+------+ It was generally noticeable that those soils found to be rich in blue-green algæ contained only a few species of diatoms, and vice versa. Diatoms appeared most frequently in soils from old gardens, whereas blue-green algæ were more characteristic of arable soils. The green algæ and moss protonema, on the other hand, were distributed universally. The majority of green algæ typically found in soils are unicellular, but a few filamentous forms occur. With the exception of _Vaucheria_ spp. these are characterised, however, by an ability to break down in certain circumstances into unicellular or few-celled fragments, in which condition identification is often very difficult. It was also found by cultural examination of a number of old stored soils from Rothamsted that germination of the resting forms of a number of algæ could take place after an exceedingly long period of quiescence. No less than nine species of blue-green algæ, four species of green algæ, and one species of diatom were obtained from soils that had been stored for periods of about forty years, the species with the greatest power to retain their vitality being _Nostoc muscorum_ and _Nodularia Harveyana_. II. THE SOIL AS A SUITABLE MEDIUM FOR ALGAL GROWTH.