Cycads and Ginkgo

The gymnosperms are so named because of their naked seeds. But before we can talk about that we have to look at the life cycle a bit first.

The gymnosperm plant is a sporophyte. It has sporophylls that produce sporangia and they come in two types: Mega- and Micro- as in all heterosporous life cycles.

The megasporophyll produces an integument around the megasporangium (which is sometimes called the nucellus). The megasporangium contains a megasporocyte which divides by meiosis to make four megaspores. Three degenerate, leaving one functional megaspore. The megaspore divides into a multicellular megagametophyte with archegonia containing an egg. This all sounds pretty standard. What makes it interesting is that all of this occurs inside the megasporangium and inside the integument. This whole structure is called an ovule throughout this part of development. Notice how the megagametophyte is no longer shed to the environment but is held inside multiple layers. It no longer needs free water! It no longer must fend for itself on the shady forest floor. It can get its nutrition from the big sporophyte. It is thus well-adapted for life on land and thus explains how cycads and other gymnosperms became dominant over lower vascular plants during the age of dinosaurs.

This evolution comes with a cost. Let's look at the male side of the house. The microsporophyll produces a microsporangium. The microsporangium contains microsporocytes. These each divide into four microspores. As on the female side of the house, the microspores are not shed. The spore divides to make a four-celled microgametophyte within the spore wall. This endosporic microgametophyte is called a pollen grain. The microsporangium opens and sheds the pollen grains...usually to the wind.

The pollen is carried by the wind to the integument of the ovule on the megasporophyll. Fortunately the integument has a tiny opening, the micropyle. The nucellus may produce a drop of liquid (the pollination droplet) to trap the pollen and suck it into the micropyle. The microgametophyte is sitting against the megasporangium (nucellus). The microgametophyte (pollen) must germinate and digest its way through the sterile jacket to get to the archegonium and egg inside. Once there, the sperm are shed, unite with the egg, and a new sporophyte can be formed.

The zygote develops into a sporophyte embryo which becomes temporarily dormant. If you remember this embryo is embedded in a nutrient-rich megagametophyte, and surrounded by nucellus and integument. When the embryo becomes dormant, the ovule is then called a seed. To have a seed you need three parts: a seed coat (the integument), storage tissue (the megagametophyte), and a dormant embryo. This seed if you recall is sitting on top of a megasporophytll, exposed to the environment. The seed (-sperm) is thus naked (gymno-); the plant is a gymnosperm.

Now that we know what constitutes a gymnosperm, let's look at some examples.

Dinosaur Food -- Cycadophyta

Cycads (Cycadophyta) were dominant vegetation at the time of the dinosaurs. It is thought that they were the food of many of the early dinosaurs at least. Evidence for this are the cells of specific size and shape loaded with silicates found in the tiny fissures of fossil dinosaur teeth. The sporophyte is very much like a fern with a thick stem and long compound leaves called fronds. As the stem grows vertically in most species, they resemble tree ferns or palms (at least superficially). Common names reflect this (eg. sago palm).

Cycads are usually dioecious, so the sporophyte either produces microsporangiate cones or megasporangiate cones...not both. Thus each cycad is functionally either male or female. Some textbooks take a very restricted view of this, but for me and for my course if it looks like a gender, acts like a gender, then it is a gender. Sexual dimorphism is found in the sporophyte in my humble opinion.

Maiden Hair Tree -- Ginkgophyta

A more advanced gymnosperm is the Ginkgo (Ginkgophyta). This tree was thought to be extinct (and is extinct in the wild). Cultivated specimens were discovered in Chinese monastery gardens and have been distributed worldwide. As a tree that resists pollution, it has found a great home in urban settings.

Ginkgo trees are much like pine trees. They have woody roots, woody trunks, long shoots and short shoots. The leaves are fan-shaped with dichotomous veination, and the leaves are deciduous (abscise in the fall). The trees are dioecious like cycads. The "male" trees have microstrobili that are like tiny catkins of birch or willow. They shed pollen to the wind. The "female" trees have naked ovules with micropyles. The ovules have a fleshy integument. The ovule falls from the tree at about the time the pollen tube has arrived at the archegonium. Syngamy is believed to occur with the ovule on the ground. The fleshy integument produces butyric acid. If you have not smelled this chemical, it REALLY stinks. Spoiled milk in an almost-empty carton that has been sitting a few days at room temperature contains butyric acid. The smelly ovules make the male trees more desirable in most settings. Hopefully humans will continue to cultivate both male and female plants so these interesting plants can still complete their life cycle.