Fern fronds, or leaves, are tightly coiled when young, to protect the growing tip and subdivisions of the frond. As the lower parts of a frond expand and toughen up, they begin to photosynthesize and can support growth and expansion further up the frond, a process known as circinate vernation. Ferns (division Pteridophyta) are vascular plants that lack seeds and instead reproduce by shedding spores. The spores are produced on the undersides of the fronds in structures called sori which are clusters of spore-producing sacs. There are twenty thousand species of plant classified in the division Pteridophyta.