LICHENS 519 



fungi attacking higher plants, can be observed at the present day. 

 Incipient parasites often promptly kill their host-plant ; whereas, 

 at the other extreme, many parasites do not kill the host-plant, 

 neither, so far as our knowledge goes, do they cause the slightest 

 inconvenience, and in some instances the presence of the parasite 

 actually appears to benefit the host-plant. When parasite and host 

 have reached the point, through a long series of struggles against 

 each other, of mutually aiding each other, and doing more for their 

 welfare when working in unison than either could do as a separate 



Cladoiiia raiii^iferina. Reindeer moss 



individual, the terms " mutualism " or " symbiosis " are used to 

 express this condition of things. Instances are known where fungi 

 and flowering plants appear to have attained to this condition of 

 mutualism, and colonies of alg^e are also invariably present in the 

 tissues of certain flowering plants. 



In all probability fungi were in the first instance parasitic on 

 algcT, and through a long series of years this primitive parasitism 

 has gradually evolved into mutualism, the result being our present 

 lichen flora. 



Cladonia cornucopioides (PI. XXXIX, fig. i).— A common and 

 very beautiful lichen, growing on the ground on heaths in lowland 



