196 



VEGETABLE PHYSIOLOGY 



sitism. Others speak of reciprocal and antagonistic sym- 

 biosis, to indicate these two different kinds of association. 



One of the best known cases of symbiosis in the strict 

 sense is that of the Lichens. These are lowly organisms 

 which are epiphytic upon tree trunks, old walls, rocks, and 

 other supporting structures. They are composed always of 

 two distinct plants, an Alga and a Fungus, which are closely 

 united together to form a kind of thallus (fig. 97). The 

 relative modes of arrangement differ in different species, 

 and many algae and many fungi are found to be capable of 

 entering into such an association. The advantages which 

 result to the two constituents of the lichen are consider- 

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

 struct carbohydrate materials by its instrumentality, and 

 after their formation these are shared 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 proteins. 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 kephir 

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



FIG. 97. SECTION OF A LICHEN SHOW- 

 ING ALGAL CELLS (#) IN THE MIDST 

 OF A NETWOBK OF FUNGAL HYPHJE 



(m). (After Sachs.) 



