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, Anabsena. 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 algse, such as Protococcus or Nostoc, are 

 included in a thallus whose principal constituent is a 

 sac-fungus, whose hyphae are closely united with the 

 green cells of the algae, 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 algae with some of the 

 lower animals, e.g. Paramoecium, 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 Leguminosae, 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 



