BSC 314 Lecture Notes - Lecture 59: Namib Desert, Indeterminate Growth, Ginkgo Biloba
28 Jun 2018
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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.
Phylum Cycadophyta
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.
Phylum Ginkgophyta
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.
Phylum Gnetophyta
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
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Document Summary
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.
