Sowing And Reaping

Profound changes in climate, glaciation and finally widespread desertification are linked to the tectonic assembly of the supercontinent Pangaea. Conifers, gingkos and cycads — naked-seed, "cone-bearing" trees — and dry-tolerant spore ferns replace the dying lush Carboniferous forests. Wind carries pollen from tree to tree, a common pollination method before some animals begin to provide the service.

Cycads grow well in soils extremely low in nitrogen. Cyanobacteria, living in the roots where they induce a special layer of tissue, convert nitrogen from the air to a form usable by the cycad.