ADJUSTMENT OF ORGANISMS TO ENVIRONMENT 393 





FiG. 229. Vertical sec- 

 tion through a lobe of 

 acommon lichen 

 (Physcia stellaris), 

 showing fruiting sur- 

 face above, a, spores 

 of the fungus; b, b, 

 algal ceils, held among 

 the fungous filaments, 

 which are loosely ar- 

 ranged at c, but com- 

 pacted together to 

 rorm protective sur- 

 face at d. (After 

 Bessey). 



with a microscope. 

 It will then be at 

 once apparent that 

 the body is mainly 

 a complex of 

 branched fungus 

 filaments and that 

 the algal cells occu- 





pymg 



rjQ; 





the middle 

 stratum, are in close union with some of 

 these filaments, enwrapped by them, 

 or indented by blunt protuberances 

 from them. 



This union is for mutual benefit. We 

 have already learned that a plant like 

 this fungus, lacking chlorophyl, cannot 

 get its carbon directly from the carbon 

 dioxide of the air; and in such situa- 

 tions, there is no other adequate source 

 of supply. Through the agency of the 

 green alga, however, and by means of 

 its close attachment to the algal cells, it 

 gets carbon made up into assimilable 

 form. It furnishes the alga in return 

 shelter and protection and retains about 

 it watery solutions containing the other 

 materials for its food. The algal cells 

 have room for growth and division: 

 alga and fungus grow together, main- 

 taining constant relations, resulting in a 

 growth habit by which lichen species 

 are known. The combination is an 



efficient one for meeting hard conditions of life in drv and 



sterile situations. 



