The rôle of the young lucerne plant in determining the infection of the root by the nodule-forming bacteria

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Abstract
The appearance of nodules on seedlings of lucerne (Medicago sativa L.) coincides with the opening of the first true leaf. There is evidence that before this leaf opens the nodule bacteria do not as a rule infect the root hairs. The delayed infection is due to the plant and not to any delay in the development of infective power by the bacteria. When young inoculated seedlings, whose first leaves are still closed, are grown intermingled with older plants, a considerable number of nodules will develop on them, although scarcely any are formed on control seedlings of the same age, grown by themselves. The solution surrounding the roots of seedlings whose first leaves are expanded has an influence in stimulating the appearance of nodules on younger seedlings, and increases the growth of the nodule organism on agar. The solution surrounding the roots of the younger seedlings has no such effect. The active substance inducing nodule appearance when the first leaf opens is not formed in this leaf, since the removal of the leaves while still closed has no effect on nodule appearance.

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