200 PLANT LIFE 



itself to several kinds of algae, and vice versa. 

 In every instance, however, a specific lichen 

 results from the union of a definite fungus with 

 a particular alga if either the alga or fun- 

 gus be changed, a correspondingly different 

 " species " of lichen is formed. 



Both organisms thrive. The algal cells often 

 become unusually large, and the fungal 

 mycelium is evidently well nourished. But 

 multiplication of cells and consequent growth 

 is often greatly modified, especially in the 

 more specialised lichens in which the two 

 organisms become more intimately dependent 

 on each other. 



The symbiosis only continues to pay as 

 long as the alga is properly exposed to light, 

 and for so long as it is properly supplied 

 with water, together with the small amount of 

 mineral food it requires. The latter offices 

 are largely discharged by the fungus, which 

 usually attaches the lichen to the substratum, 

 whilst its gelatinous walls retain the water 

 supplies derived from intermittent showers 

 or other sources. Thus a remarkable degree 

 of correlation is displayed in the growth 

 processes of the specialised lichens, and some 

 of them simulate to a wonderful extent the 

 form, and partly even the structural arrange- 

 ments, to be met with in the green leafy 

 shoots of higher plants (Figs. 23A and B). 



Lichens are particularly instructive in 

 showing that the form assumed by an organ- 

 ism is in the long run determined by the 



