OTHEE METHODS OF OBTAINING FOOD 211 



able. The alga, which possesses chlorophyll, is able to con- 

 struct carbohydrate materials by its instrumentality, and 

 these are shared after their formation by the fungus, 

 which has no such construc- 

 tive powers. The fungus is 

 able to condense aqueous 

 vapour, which is very neces- 

 sary in the dry situations 

 lichens occupy. It can thus 

 dissolve much of the dust 

 and other debris of its rest- 

 ing place, and so carry raw 

 material to the constructive 

 algal cells. It also attaches 

 the thallus to the substratum. 

 Both partners can no doubt 

 take, part in the construc- 

 tion of proteids. The rela- 

 tionship affords a further 

 advantage, for the compound 

 organism is much better 



able than either of its separate constituents to resist 

 adverse conditions of temperature, drought, &c. 



A similar symbiosis is met with in the so-called Jcephir 

 organism and others of the same kind. In these cases the 

 two constituents are a yeast and a bacterium, the former of 

 which is closely surrounded by chains of the latter, making 

 a fleshy mass of irregular shape, and sometimes of compara- 

 tively conspicuous dimensions. The parts played by the two 

 organisms are not very well understood, but there seems to 

 be no doubt that the association is mutually beneficial. 



In a former chapter mention was made of a property 

 which is possessed under certain conditions by various 

 plants belonging to the Leguminosce that of being able to 

 utilise the free nitrogen of the air in the construction of 

 proteid food-substances. The power was shown to be con- 

 nected with the formation of certain tubercular structures 



p 2 



FIG. 101. SECTION OF A LICHEN SHOW- 

 ING ALGAL CELLS (g) IN THE MIDST 

 OF A NETWORK OF FUNGAL HYPH^E 



(m). After Sachs.) 



