86 BOTANY. [chap. ii. 



together constituting the individual. The algal element, 

 being possessed of chlorophyll, assimilates carbonic 

 dioxide and forms organic carbon compounds, while 

 the mycelium of the fungus portion absorbs water con- 

 taining mineral substances in solution. The mycelium 

 of the fungal portion of the lichen clasps the cells of the 

 alga so closely that a transfusion of the substances 

 absorbed by the two respectively takes place ; but this 

 differs from a case of true parasitism, inasmuch as alga 

 and fungus mutually benefit. The fruit of every lichen 

 is formed entirely by the fungal element. The forma- 

 tion of a new lichen depends on the spores produced by 

 previous lichen germinating in contact with an alga the 

 cells of which are clasped by the mycelium of the germi- 

 nating spore, and growth or increase in size is effected 

 by the independent and contemporaneous growth of the 

 two individuals. This perfect balance and readjustment 

 of the division of labour between two originally distinct 

 plants to form a third possessing pronounced peculiarities 

 of its own, with a corresponding loss of individuality of 

 its -components, is a marked illustration of the adapta- 

 bility of life, and is not an isolated example, as mutualism 

 in every phase of development exists in various divisions 

 of both the animal and plant kingdoms, and well authenti- 

 cated instances of mutualism between plants and animals 

 are also on record. 



(2) Saprophytes. Plants that obtain their food from 

 dead organic matter, and are always destitute of chloro- 

 phyll, or at all events possess such minute traces as to 

 be of no functional value in assimilation, but is of interest 

 as one of the proofs of their having degenerated from 

 chlorophyll-bearing ancestors, other points of evidence 



