Gymnosperms and angiosperms

Gymnosperms (gymnos = naked, sperma = seed) have ovules that are not enclosed in an ovary, so their seeds are naked. They are trees and shrubs, some very large: the giant redwood Sequoia is one of the tallest trees in the world. Think of pine (Pinus) and Cycas.

Their roots are tap roots. Pinus roots live with fungi (mycorrhiza). Cycas has special coralloid roots in which cyanobacteria fix nitrogen. Cycas leaves are large and feather-like (pinnate). In conifers (like pine), leaves are needles with a thick cuticle and sunken stomata, which reduce water loss.

Gymnosperms are heterosporous. Male cones (strobili) make microspores, which develop into very reduced male gametophytes called pollen grains. Female cones bear ovules; inside, a megaspore develops into the female gametophyte with archegonia. Pinus carries both kinds of cone on the same tree; Cycas on different trees.

Unlike bryophytes and pteridophytes, the gametophytes are not free-living: they stay inside the cones. Wind carries pollen to the ovule. A pollen tube then carries the male gametes to the archegonia, so no water is needed. The zygote grows into an embryo and the ovule becomes a naked seed.

Angiosperms (flowering plants) produce pollen and ovules inside flowers, and their seeds are enclosed in fruits. Any plant you have seen flowering, and any fruit you have eaten, comes from an angiosperm. They range from the tiny Wolffia to Eucalyptus trees over 100 metres tall. They are divided into two classes: dicotyledons and monocotyledons.