In a paper communicated to the Royal Society in December, 1898, an account was given by the writer of various experiments bearing on the nature of Leguminous nodules and the organisms concerned in their formation. The work, there described, had reference almost exclusively to the nodules borne by Pisum sativum and Vicia hirsuta , but it is now proposed to record some further results obtained from a more comparative study of nodules from other genera of the Leguminosse, and, in addition, to discuss, in some detail, subsequent experiments upon the nodule organisms, both in connection with their behaviour on artificial media, and the effect of their action upon the host plants, and, in conclusion, to consider these experiments in relation to the intimate connection, which they suggest as existing, between the host plants and their parasitic organisms on the one hand, and the very varied and complicated physical and biological factors of the environment on the other hand. As regards anatomical characters, the facts to hand are still not sufficiently comprehensive to admit of any general conclusions being drawn as to the relation of the nodules borne by the genera of the different sub-orders of the Leguminossæ ; it will, therefore, be convenient to consider each genus separately, irrespective of its systematic position in the order.