394 



GENERAL BIOLOGY 



Some species that live symbiotically can be cultivated 

 apart ; but others appear to have become so fully established 

 in this manner of life that they are no longer able to live 

 apart. 



There are other cases of symbiosis in different groups. 

 We have already seen green hydras; the color is due to 

 minute algal cells (zoochlorellcE) living within the larger cells 

 of the hydra, doubtless using there the carbon dioxide which 

 the hydra cells excrete, and giving them back again the 

 liberated oxygen for respiration. Attached to the roots 

 of beech tree s are molds that do for the tree the absorbing 

 work ordinarily performed by rhizoids, while the tree sup- 

 plies them with carbon products. Thus here also the benefit 

 is mutual. 



Sttidy jo. The relations of fungus and alga in the lichen. 



Materials needed: Lichens of the two types shown in 

 figure 227, the foliose one with spore cupules (apothccia) 

 developed. Razor and pith for cutting sections. 



Place a cupule-bearing thallus between two wet pieces of 

 pith, and cut vertical sections as thin as possible wdth a 

 razor. Mount and examine a number of these and select 

 the best for study. 



Mark the general arrangement and distribution of 

 fungus and alga. Then study the fungus: 



i) The form of its filaments in the several layers. 



2) The form of its fructification in the cupule; compare 

 with an account of the Ascomycetes, in any good text -book 

 of botany. 



Then study the alga. To do this remove the cover, tease 

 the algal layer of a section to bits on a slide, cover again, 

 and study the alga in the fragments. Determine the rela- 

 tions to the algal cells of the investing fungus filaments. 

 Look for evidences of division in the algal cells. 



