203 



the lichens. As has been seen above, a lichen con- 

 sists of two distinct plants, viz. an alga and a fungus, see page 

 137. The fungus receives carbohydrates from the alga in 

 return for which it protects the alga from drought and pro- 

 vides it with water and mineral salts. 



179. Another well-known exam- Ba cteriaand 



pie is furnished by the bacteria which, as noted in Part IV Leguminosae. 

 above, see page 129, live in the tubercles often found on the roots 

 of the Pea, Bean and allied plants belonging to the Natural 

 Order Leguminosae. The best known of these bacteria is named 

 Bacillus radicicola which, passing through the root hairs, 

 gives rise to the peculiar tubercles in the root cortex of the 

 infected plants. The bacteria are localized in these tubercles 

 and do not spread to neighbouring tissues. While the bac- 

 teria live on the carbohydrates supplied by the host they, 

 in their turn, are able to provide the host with valuable 

 nitrogenous food materials, owing to the power possessed by 

 them of fixing the free nitrogen of the air and of manufac- 

 turing from it a compound which can be utilized as food by 

 higher plants. Owing to this remarkable symbiosis by means 

 of which most leguminous plants can accumulate nitrogenous 

 organic material in their tissues, a crop of such plants can 

 actually enrich a soil poor in nitrates. Hence in agriculture 

 it is a common practice to grow such plants and to plough 

 in the crop as manure.* In a soil rich in nitrates, leguminous 

 plants can obtain their nitrogenous food without the aid 

 of these bacteria, and in such cases very few, if any, tubercles 

 are formed. 



180. The majority of the Mycorhizae . 



higher plants cannot directly utilize as food the organic 

 substances, such as the dead remains of plants, wood, leaves, 

 etc., which compose the humus of the Forester. Hence 

 such plants are found to grow normally when supplied 

 solely with water and the necessary mineral salts. Many 

 plants, however, such as pines, oaks, the hazel and others, 

 can thrive on humus soils, and in the majority of such 

 plants their roots are found to be living in symbiosis with 

 the mycelia of fungi. The fungal hyphse are sometimes 

 found chiefly inside the cells of the root cortex, a few fila- 



*lf leguminous plants could be grown continually on the same soil the supply 

 of nitrogen, in the latter might clearly be increased indefinitely, but unfortu- 

 nately this is not at present possible. Experiments in Europe have shown that 

 clover can only be successfully grown on the same soil about once in four 3 7 ears 

 and sometimes even not so frequently. The reason for this is at present unknown. 



