+-------------------------+----------------------------------------+ | | Percentage of Samples | | | containing given Alga. | | +--------------------+-------------------+ | | Uncultivated | Cultivated | | | Damp Sandy Soil. | Soils. | | +------+------+------+------+-----+------+ | |Shores|Shores| | | | | | | of | of | Sea- | | |Marsh-| | Species. | Elbe.|Lakes.|shore.|Sandy.|Clay.| land.| +-------------------------+------+------+------+------+-----+------+ |Anabæna variabilis | 46 | 43 | 9 | 10·3 | 60 | 46 | |Anabæna torulosa | 31 | 14·3 | 63·6 | 27·6 | 34·3| 56·4 | |Cylindrospermum muscicola| 23 | 28·6 | 0 | 24 | 48·6| 59 | |Cylindrospermum majus | 0 | 14·3 | 0 | 38 | 40 | 33·3 | |Nostoc Sp. III. | 7·7 | 0 | 0 | 38 | 37 | 48·7 | +-------------------------+------+------+------+------+-----+------+ Taking the number of samples containing blue-green algæ as a rough measure of their relative abundance, Esmarch obtained the following interesting figures (Table XI.):-- TABLE XI. +------------------------------+----------------+---------+ | | Percentage | | | | of Samples |Number of| | | Containing | Samples | | Kind of Soil. |Blue-green Algæ.|Examined.| +------------------------------+----------------+---------+ | | | | |Cultivated marshland | 95 | 40 | |Cultivated clay soil | 94·6 | 37 | |Uncultivated moist sandy soils| 88·6 | 35 | |Cultivated sandy soil | 64·4 | 45 | | {Woodland | 12·5 | 40 | |Uncultivated{Sandy heathland | 9 | 34 | | {Moorland | 0 | 35 | +------------------------------+----------------+---------+ In noting that the soils fell into two groups, those relatively rich and those poor in blue-green algæ, Esmarch concluded that the two chief factors governing the distribution of the _Cyanophyceæ_ on the surface of soils are, (1) the moisture content of the soil, (2) the availability of mineral salts, cultivated soils being especially favoured in both of these respects. He further distinguished between cultivated land of two kinds, viz. arable land and grass land, and found that on all types of soil grassland was richer in species than was arable land.