232 PHYSIOLOGY 



lichens have their habitat on the sea-shore, constantly subject to spray from 

 the waves, but scarcely any can exist within the spray of a waterfall, 

 possibly because the latter is never-ceasing. 



B. Storage of Water 



The gonidial algae Gloeocapsa, Scytonema, Nostoc, etc. among Myxophy- 

 ceae, Palmella and occasionally Trentepohlia among Chlorophyceae, have 

 more or less gelatinous walls which act as a natural reservoir of water for 

 the lichens with which they are associated. In these lichens the hyphae 

 for the most part have thin walls, and the plectenchyma when formed — as 

 below the apothecium in Collema grcmuliferum, or as a cortical layer in 

 Leptogium — is a thin-walled tissue. In lichens where, on the contrary, 

 the alga is non-gelatinous — as generally in Chlorophyceae — or where the 

 gelatinous sheath is not formed as in the altered Nostoc of the Peltigera 

 thallus, the fungal hyphae have swollen gelatinous walls both in the pith 

 and the cortex, and not only imbibe but store up water. 



Bonnier' had his attention directed to this thickening of the cell-walls 

 as he followed the development of the lichen thallus. He made cultures 

 from the ascospore of Physcia {Xanthorid) parietina and obtained a 

 fair amount of hyphal tissue, the cell-walls of which became thickened, 

 but more slowly and to a much less extent than when associated with the 

 gonidia. 



He noted also that when his cultures were kept in a continuously moist 

 atmosphere there was much less thickening, scarcely more than in fungi 

 ordinarily ; it was only when they were grown under drier conditions with 

 necessity for storage, that any considerable swelling of the walls took place. 

 Further he found that the thallus of forms cultivated in an abundance of 

 moisture could not resist desiccation as could those with the thicker 

 membranes. These latter survived drying up and resumed activity when 

 moisture was supplied. 



C. Supply of Inorganic Food 



As in the higher plants, mineral substances can only be taken up when 

 they are in a state of solution. Lichens are therefore dependent on the sub- 

 stances that are contained in the water of absorption : they must receive their 

 inorganic nutriment by the same channels that water is conveyed to them. 



a. FOLIOSE AND Fruticose Lichens. These larger lichens are provided 

 with rhizinae or with hold-fasts, which are only absorptive to a very limited 

 extent; the main source of water supply is from the atmosphere and the 

 salts required in the metabolism of the cell must be obtained there also — 



1 Bonnier i88g^ 



