Primitive seed plants - cycads, Ginko, and the gnetophytes
There are a few genera of seed plants that appear to
have survived from the Age of the Dinosours. Expert opinion has changed
many times about the exact position of these relics of an ancient past
in the evolutionary scheme. It is currently accepted that the Cycadophyta
and the Ginkophyta are naked-seeded (gymnosperms) distantly related to
other gymnosperms such as Pinophyta, and that the Gnetophyta are more closely
allied with the flowering plants (angiosperms). We will study them
only briefly as an introduction to the seed plants. They help us
bridge the vast gaps between the structures of the ferns and the conifers
and flowering plants
Ginkophyta
Ginko biloba, the only living species of the
division, is a popular ornamental in the Orient and in urban settings of
the northeastern United States. This ancient tree grows to a tremendous
girth, over its life span of many hundred years. Ginko has
secondary growth comparable to gymnosperms. It adds layer after layer
of xylem and phloem to make a strong trunk and root.
We have several Ginko biloba trees.
Because they are deciduous, some are in refrigeration, in a dormant stage.
By taking out different trees each term it is possible to have specimens
in full foliage at any time. The trees in the greenhouse are in full
foliage, those outside are about to lose their leaves. The tree gets its
common name "maiden hair tree" from the shape of the leaves, which resemble
the hairdos of unmarried Japanese girls, of the upper classes, in the feudal
period. Note the dichotomous venation.
Ginko trees are dioecious. "Female"
ginkos produce two ovules (encased megasporangia) on a short determinate
branch. It produces many microsporangia on stalks, in clusters almost
dense enought to be considered strobili.
The seed that results from pollination of the ovule
and subsequent fertilization of the egg is covered with a pulpy material
that has a vile odor when mature. For this reason, "male" trees are
preferred for ornamental plantings.
Cycadophyta
There are ten or eleven genera of cycads containing
about one hundred species. Most cycads grow in dry, tropical areas.
Only two species of Zamia grow in the continental United States,
both in Florida. There are several species in Mexico.
Cycads are notable for short, stout stems and massive,
compound leaves. The leaves are noted for having sharp hard tips.
You will, undoubtedly, get stuck a few times in the greenhouse. Move
carefully. Unlike the Ginko, most of the stem of the cycads
is pith and leaf base. The secondary growth is light. Adventitious
buds, called "pups" sometimes form on healthy cycads. They are evident
on some of our larger specimens.
The cycads have two forms of root. The coralloid
roots are large spongy masses that house many symbionts, including
many cyanobacteria.
Cycads are useful for studying the evolution of
the strobilus (cone) in gymnosperms. The strobili of the cycads are
generally large, bordering on the spectacular. The megasporangiate
cones are larger than the microsporangiate cones. In the most primitive
case, the megasporphylls are arranged, on the stem, in a loose spiral just
above the vegetative leaves. A new ring of vegetative leaves appears
above the sporophylls, then a new ring of sporophylls. Check the
pattern on the big Cycas revoluta in the dome. Most cycads
have developed specialized branches on which the sporophylls are highly
reduced to form bracts. These are the cone-like strobili. There
are several examples in the temperate room in the greenhouse. These
are called simple strobili, in contrast to the compound strobili that we
will see in the Pinophyta.
Gnetophyta
Whereas the ginkophytes and cycads are considered primitive
gymnosperms, the gnetophyta appear to be related to the ancestors of the
flowering plants. The outstanding difference betwen the gnetohytes
and the flowering plants is that the gnetophytes have cone-like strobili.
Otherwise, they share several anatomical characteristics and some reproductive
charateristics with the flowering plants.
We are fortunate to have examples of all three genera
of the gnetophytes: Gnetum, Ephedra, and Welwitschia.
Gnetum is worth a look, just to see how much like a flowering plant
it looks. If we are lucky enough to see any of the reproductive structures
we will see that it is not an ordinary flowering plant.
Ephedra is an inconspicuous plant that could
easily be mistaken for Equisetum.. It should be in the desert
room and outside. Look for small cones that look almost like
buds, at the joints along the stem. Ephedra is the original
source of ephedrine, a heart stimulant. An American species was known
as Mormon Tea. We will collect some young cones to dissect.
Welwitschia is a wonder. A native of
the coastal deserts of southern Africa, it can develop an enormous tap-root,
although the stem never gets much above the ground. It has only two
leaves that persist for the life of the plant. The leaves have an
intercalary meristem. As the wind and the sand wear off the tips
of the leaves, the basal meristem keeps producing more leaf.
The HSU greenhouse has some of the finest specimens
of Welwitschia in captivity. They produce strobili regularly.
Probably less than one one-thousandths of one percent of the people in
America have seen Welwitschia, and only a few of those have seen
the strobili. How privileged can you get? Please, don't touch
theses valuable structures
Key words: dioecious, secondary growth, ovule, deciduous, adventitious,
coralloid roots