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KINGDOMS MONERA AND PROTISTA: 



the case for the alga. For example, when lichens 

 occur in environments where the alga could not sur- 

 vive alone, both plants are obviously benefiting. 

 However, in other instances, it is not obvious that the 

 alga is helped. This appears to be the case when an 

 alga is both free-living and, within inches, confined by 

 a fungus. At the very least, the alga could live alone, 

 but in such a case the fungus might even be a para- 

 site. The fungus may be shading the alga, hence 

 retarding alga photosynthesis, and may be utilizing 

 alga nutrients. 



Lichen reproduction is often the consequence of 

 normal fungal reproduction in which developing 

 fungal filaments probably entrap algae; however, 

 within a lichen, algal reproduction normally is 

 limited to asexual mitosis. On the other hand, 

 there is a process by which lichens reproduce lichens 

 directly. This asexual mechanism features the for- 

 mation of a small fragment of lichen composed of a 

 few algal cells entrapped within a mycelium. Such re- 

 productive structures are formed inside the lichen but 

 grow to the surface, become ball-like, detach, and 

 usually are carried by the wind until a proper site is 

 reached for the new lichen to grow. 



Because lichens can become dormant, they are able 

 to withstand severe, periodic drought. For this 

 reason they are important early inhabitants of rocks 

 and are extremely widespread. There are few areas 

 in the world where lichens cannot be found. They oc- 

 cur in hot dry, cool moist, and even frozen en- 

 vironments. Frequently they grow upon objects, es- 

 pecially living and dead plants. 



Although the combination of algae and fungi con- 

 stitutes a unique growth form with functions all its 

 own, lichens are not a natural grouping of organisms. 

 This does not mean that the lichen-forming fungi, 

 especially, show little specialization or evidence 

 of evolution toward the lichen way of life. Actually, 

 such evolution did occur and must have taken some 

 time. However, evolution was not of single lichen 

 species; rather, remotely related fungi and "algae" 

 were involved. Therefore, any classification of the 

 group as a whole has no phylogenetic meaning. For 

 this reason, classification of lichens is strictly for 

 the convenience of specialists, and the growth-form 

 segregation of lichens into crustose, foliose, and 

 fruticose types is sufficient for most naturalists 

 (Figure 8.17). 



Crustose lichens grow in thin crusts and usually 

 occupy the driest habitats. Foliose lichens often are 



crustose 



fohose 



fruticose 



Figure 8.17 Lichen types, about natural size, but many kinds are 

 much larger. Crustose and foliose types are shown from above, fruti- 

 cose from the side. 



somewhat encrusting but are more elevated from the 

 substrate, generally display a pattern of two-forked 

 branching, and sometimes vaguely resemble leaves. 

 They generally require more water than do crustose 

 but less than fruticose lichens. Fruticose lichens 

 are upright or hanging, and often are confused with 

 and called mosses. 



SELECTED READINGS 



Alexopoulos, C. J., 1962. Introduclun Myrnlogy. 2nd ed. 



John Wiley & Sons, New York. 

 Christensen, C. M., 1946. Common Fleshy Fungi. Burgess 



Publ. Co., Minneapolis, Minn. 



