Other Living Gymnosperm Phyla
The surviving gymnosperm are a diverse group that persist today in restricted habitats or in regions too extreme—too hot, too dry, too cold—for angiosperms. Some preserve in their structures and life‐styles evolutionary early “flowers” that didn't quite succeed and the experimental lifestyles later adapted and adopted by the angiosperms.
The cycads look like palms with cones and are prevalent worldwide in the tropics and subtropics (two species grow wild in Florida). Some grow 15 meters or more in height, but many have shorter trunks and an almost rosette appearance. The ovulate cones are large (some weigh over 30 kilograms) and are borne upright on megasporophylls among the vegetative leaves. Pollen and ovulate cones are produced on different plants. Despite the un‐pine‐like appearance, the life cycle of the cycads is similar to the pines. The sperm, however, though carried to the archegonia in a pollen tube, are multiflagellate with hundreds of flagella.
Zoologists consider the Jurassic Period the “Age of Dinosaurs,” but botanists refer to it as the “Age of Cycads.” Cycads and bennettitaleans, an extinct group of plants that often are mistaken for cycads because of the close resemblance of their leaves and growth form, dominated the land flora.
Another un‐pine‐like gymnosperm is Ginkgo biloba, the maidenhair tree, the sole remaining representative of a group of important plants of the Mesozoic forests. It has broad, fan‐shaped leaves with dichotomously branching veins and is deciduous. It is widely planted as a street tree because it withstands well the air pollution of cities. Its seed coats, however, have a foul odor, and when the seeds fall around the trees and rot in the heat of summer, it becomes a much less desirable plant. There are no known wild Ginkgos; the plants of today derive from stock preserved in temple gardens by monks in China and Japan. They distributed seeds to gardeners around the world over 200 years ago.
Several features unite Ginkgo with the rest of the gymnosperms, but to which precise lineage—the conifer line or the cycad—is still being debated. No cones are produced and the female gametophyte is contained in a cherry‐like seed. The ovules and microsporangia are produced on different trees.
Three living genera—none of whom resemble one another or any other living gymnosperm—constitute the Gnetophyta, Gnetum, Ephedra, and Welwitschia. The gnetophytes are the closest living relatives of the flowering plants, and they form a monophyletic clade. Gnetum species are tropical vines and trees that resemble flowering plant species with their broad, simple leaves. Ephedra, called joint‐fir or Mormon tea, is a desert shrub with worldwide distribution. The species of Ephedra have green, jointed stems and small scale‐like leaves. They produce several secondary metabolites that are chemically similar to human neurotransmitters, and people have used the plants as medicinal teas for centuries. Welwitschia, among the weirdest of plants, is confined to the Namib Desert of southwestern Africa and has a buried trunk on which two strap leaves of indeterminate growth are attached. The leaves split lengthwise into strips and blow about on the shifting sand. Ovulate and pollen cones are produced on separate plants on the rims of the exposed trunks. The eggs in the megagametophyte move towards the pollen tubes in their own tube‐like structures; fertilization takes place after the two tubes fuse.
There are few good gnetophyte fossils, the best evidence of their past occurrence being pollen that resembles that of Ephedra found in Triassic and Cretaceous strata. This peculiar collection of plants has several angiosperm characteristics, but none of the plants in the group is the direct ancestor of the angiosperms.