These are swollen and branch Dichotomously.

When they are sectioned a dark green layer is seen in the Cortex. This was called the "Algal Zone" because it contains Blue-green Algae (Cyanobacteria). These microorganisms can fix atmospheric Nitrogen. Nitrogen is typically the most "limiting element" in natural ecosystems. Plants that have symbioses like this have an adaptive advantage over other plants. Many invasive (Pioneer) species have N-fixing symbioses. 

Cycads are not invasive. However, they often grow in ecologically challenging or 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.

There are few Cyanobacterial symbioses with vascular plants. Some Hornworts, the aquatic fern Azolla and monocots in the genus Gunnera have similar but distinct symbioses with Cyanobacteria. A species of Gunnera grows in Hawaiian forests and has been studied. However, there are still unanswered questions regarding this symbiosis. Gunnera anyone????

Cultured Macrozamia Root Nodule

Macrozamia Nodule seen with SEM