These are swollen and branch Dichotomously. When they are sectioned a dark green layer is seen in the Cortex . These microorganisms can fix atmospheric Nitrogen . Plants that have symbioses like this have an adaptive advantage over other plants.
Many invasive species have N-fixing symbioses.
Cycads are not invasive. However, they often grow in ecologically challenging, barren sites where little else can survive. Consequently, this symbiotic relationship may have been the difference between extinction and survival for the Cycadophyta. Some Cycads produce root nodules without any obvious biotic or abiotic stimulus, while others require light for nodulation. There is a lot known about nodulation but there remain a lot of unanswered, fascinating questions to be answered about this symbiosis.