The gonidial algae _Gloeocapsa_, _Scytonema_, _Nostoc_, etc. among Myxophyceae, _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 granuliferum_, 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[842] 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_ (_Xanthoria_) _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 substances that are contained in the water of absorption: they must receive their inorganic nutriment by the same channels that water is conveyed to them.