156 



PL A N T M ORPHOLOG Y 



without which the alga could not live. The fungus derives food from the 

 alga while the alga obtains moisture from the fungus. This reciprocal 

 relation makes it possible for many lichens to live in dry exposed situa- 

 tions where neither the alga nor the fungus could live alone. Thus the 

 relation between the two lichen components is one of mutual advantage. 



w^ 



V0K 



■'?,7'..:^'T 



vi^ 



;;i''l.- 



^•- ^-i-.-j^-' \;'V/( ,•''-- " -v-'.< .'.ii^ 



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Fig. 127. .4, longitudinal section through an apothecium of Physcia, showing hymenium 

 and embedded algal cells, X60; B, enlarged view of hymeniuni, showing asci and para- 

 physes, X500. 



COMPARISON OF THE CLASSES OF FUNGI 



The chief distinguishing characters of the five classes of fungi are as 

 follows : 



Schizomycetes. Plant body unicellular, solitary or colonial, ciliated 

 or nonciliated. Cells without a definite nucleus. Cell walls usually 

 forming mucilage. Reproduction by fission. 



Myxomycetes. Plant body a naked amoeboid mass of multinucleate 

 protoplasm (a plasmodium). Asexual reproduction by small uninucleate 

 spores, each with a cell wall and usually borne within sporangia of definite 

 form. Sexual reproduction by amoeboid isogametes. 



