False-colour scanning electron micrograph of a root nodule on the white clover plant, Trifolium repens, caused by the nitrogen-fixing bacteria Rhizobium trifolii. The bacteria convert atmosph- eric nitrogen into a usable organic form, something the clover cannot do itself, but which is imperative for its survival. Bacteria infect the plant through root hairs, forming an infection thread, which conveys them from the entry point to the nodule site. Here they divide repeatedly, swell up, & become known as bacteroids. The nodule consists of a central region filled with bacter- oids surrounded by a spongy region, the cortex, seen here. Magnification: x6 at 35mm size.

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