By the use of 15N the occurrence of fixation of elemental nitrogen has been demonstrated in the lichens Collema granosum and Leptogium lichenoides (both of which contain Nostoc as a symbiont), and also in the liverwort Blasia pusilla (which has Nostoc-containing cavities in the thallus). The circumstances indicate that the fixation in these instances should be attributed to the Nostoc. No evidence of fixation was obtained in similar isotopic tests on the mycorrhizal roots of intact plants of Calluna vulgaris and of Pinus sylvestris, although these, more especially in the case of Calluna, have been held by some previous authors to be nitrogen-fixing.