CHAP. VHI.] SYMBIOSIS AND PARASITISM. 65 



Chapter VIII. 

 SYMBIOSIS AND PARASITISM. 



SYMBIOSIS (Greek: syn, together; bios, life) is a term applied to 

 the >tate of affairs which arises when two different organisms 

 live in company, neither harming the other, but each on the con- 

 trary obtaining benefil from its partner's activities. One of the 

 most familiar examples of this interrelationship is afforded by the 

 numerous tribe of lichens which occur so commonly on rocks and 

 trees in all damp places. Until comparatively recently the lichens 

 were considered to be a distinct class of plants but it is now known 

 that a lichen is not one distinct organism but a compound of two 

 plants, a fungus and an alga, utterly distinct in structure and in 

 mode of life. Owing to the absence of chlorophyll a fungus can 

 onlj obtain tin- carbon necessary to its growth by appropriating the 

 tissues alreadj elaborated by a chlorophyll-containing (green) plant 

 for its own use and it can therefore live only on dead or living 

 Organic tissues. In tin case of a lichen, the fungus absorbs water 

 and other material from the bark or rock on which the lichen is 

 growing and can use as nutriment those substances which have 

 formerly been part of living organisms but the non-organic mate- 

 rials are passed on to the algal partner which is able to utilize 

 them by means of its green cells. The alga and the fungus each 

 make use of what the other cannot utilize and the waste, left over 

 I) each as the result of its vital activity, is again assimilated bj 

 the other partner. Thus the association is complete and mutually 

 advantageous. 



There are many degrees of symbiosis and it is not always 

 necessary that the partners should live permanently in company as 

 in the case of lichens. The association may be intermittent although 

 the benefit is mutual. A common example of this state ol things is 

 seen in the case of cattle egrets and other birds which attend 

 cattle and catch the flies and ticks which attack these animals. In 

 this case the birds derive benefit by feeding on the flies whilst the 

 cattle benefit at the same time by being rid of their parasites. 



The term commensalism (Latin; cum, together with, in com- 

 panj with ; mensa, a table) is often used as a synonym of symbio- 

 sis, but it is better to restrict this term to cases of symbiotic rela- 

 tionships in which the common partners share tin' same food, as it 

 seems obviously incorrect to apply the term commensalism to cases 

 5 



