280 EVOLUTION OF PLANTS 
Anthoceros, and others, always have within the thallus 
colonies of a low blue-green alga, Nostoc, and the 
little water fern, Azolla, has in each leaf a cavity con- 
taining a colony of a similar alga, Anabena. Just 
what are the mutual relations of the plants in these 
cases has not been clearly made out. 
Somewhat different is the case of the lichens, where 
various low alge, such as Protococcus or Nostoc, are 
included in a thallus whose principal constituent is a 
sac-fungus, whose hyphe are closely united with the 
green cells of the alge, and which is incapable of de- 
velopment if the algal cells are absent. The latter, 
however, grow perfectly well when removed from the 
lichen thallus, and it is doubtful whether they benefit, 
to any great extent, from their association with the 
fungus, except as they are sheltered and perhaps pro- 
tected from excessive drying. Somewhat similar is the 
association of minute unicellular alge with some of the 
lower animals, e.g. Paramecium, Spongilla, Hydra, etc. 
Among the most important cases of symbiosis are 
those existing between various organisms in the soil 
and the roots of flowering plants. The most noteworthy 
of these organisms are the nitrifying bacteria which are 
the principal agents in the preparation of nitrogenous 
matter in the soil, so that it is available for the higher 
plants. These peculiar organisms sometimes associate 
themselves directly with the plants, this being espe- 
cially noticeable in the Leguminose, which are notably 
rich in nitrogen. In these, e.g. pea, clover, lupine, 
etc., there are developed upon the roots little tubercles 
within which are great numbers of minute bacteria, to 
whose activity is due the assimilation of nitrogenous 
