Gymnospermae

Pteridosperms [continued from Chapter 4]

We are continuing our survey of pteridosperms from Chapter 4 [Box 4.2] with a sub-group typified by a fossil plant called Calamopitys. This sub-group can only tentatively be placed in the pteridosperms because its vegetative parts are the only fragments that have ever been found. However, their basic structure and the features of their wood are so characteristic of the pteridosperms that there is no good case for placing them in a sub-group of their own. Some genera exist only as parts of plants such as petioles. All are extinct.

Examples:
Calamopitys, Stenomyelon, Kalymma, Chapelia, Diichnia.

A sub-group that generated a lot of interest at the time of its discovery is based on the genus Caytonia. The name was given to bunches of fossil ovules that were almost totally enclosed in curved, leaf-like structures called cupules. However, careful examination showed that the ovules were not totally enclosed and therefore were still at the gymnosperm level. They were not angiospermic as was at first thought. Detached leaves are assigned to the genus Sagenopteris and pollen-bearing organs to Caytonanthus.

There is a small sub-group whose fossil remains are only known from Gondwanaland. That is the name given to the southern hemisphere portion of the super-continent Pangaea. The latter existed from about 280 million years ago until it started to break up about 140 million years later. The fossils of this sub-group are found distributed through present-day South Africa, Australia, South America, and India. These fossil plants illustrate well a problem that is commonly encountered in reconstructing their appearance as entire plants when they were living.

All of their various plant parts have never been found attached to one another but they consistently occur together, the cuticles on the separate parts are identical and pollen grains found in pollen sacs also occur around the ovules. It seems reasonable, then, to assume that we are dealing with one plant.

The leaves are classified in the genus Dicroidium; Umkomasia and Pilophorosperma are seed-bearing fragments; Pteruchus is a seed-bearing portion. Rhexoxylon was a substantial tree-sized plant with a trunk half a metre in diameter and consistently associated with Dicroidium leaves.

A rather larger and certainly much more important sub-group than some of the smaller ones just described is based on the genus Glossopteris. These plants were widely distributed throughout Gondwanaland. Fossil remains of Glossopteris indicate that members of this genus attained the stature of trees. With their large (up to 30cm long) tongue-shaped leaves (hence the name Glossopteris) they were very handsome plants. Another genus in this sub-group was Gangamopteris. The seeds and pollen-bearing organs appear to have taken various forms and no single example is typical. Their isolated roots, with their peculiar internal cavities, are classified in the genus Vertebraria.

Cycads and cycadeoids

We come now to some sub-groups of gymnosperms that are quite distinct from one another and cannot be loosely collected together under one heading as was possible with the pteridosperms.

The cycads are a small sub-group of, mainly, palm-like plants, ten genera of which still survive today. They have a long fossil history, the earliest appearing in rocks about 280 million years old. Some fossil cycads bore ovules around the margins of modified leaves as is the case with the living genus Cycas. Other fossil genera had their ovules in cones such as are seen in the living Zamia. The pollen-bearing organs, though, or all genera both fossil and living are usually borne in cones.
The stem anatomy of cycads is very characteristic and can be used to distinguish fossil cycad trunks from other fossil tree trunks. The wood of a cycad trunk has a rather open structure, unlike those of conifer and hardwood trees with their trunks of wood, solid almost to the very centre.

Fossil examples:
Phasmatocycas, Bjuvia, Beania, Archaeocycas, Leptocycas.

Fossil cycad leaves are classified in the following genera:
Nilssonia, Deltolepsis, Ctenis, Pseudoctenis, Macrotaeniopteris, Doratophyllum.

The cycadeoids bear a number of similarities to the cycads but can be distinguished from them by two essential characters. The anatomy of the trunk wood has already been mentioned. The second feature concerns the structure of the stomata; microscopic examination shows that the stomatal apparatus of the two groups developed in quite different ways.
Cycadeoidea had a short, squat trunk with a crown of spirally arranged, palm-like leaves. The reproductive organs consisted of a central female cone with ovules, surrounded by a whorl of leaf-like structures on which were borne the pollen-bearing organs [Fig. 20]. It has to be said, though, that in spite of the beautiful reconstruction, it may well be that the whole complex arrangement never opened in this way but that the pollen was shed internally on to the ovules. However, this flower-like structure created a lot of interest when it was first discovered and the cycadeoids remain the sub-group most likely to have given rise to the flowering plants.

Williamsonia was another member of this sub-group. This had separate ovule and pollen-bearing structures whilst those of Cycadeoidea were combined.

Leaf genera in this sub-group are as follows:
Nilssoniopteris, Pseudocycas, Zamites, Pterophyllum, Ptilophyllum, Otozamites, Dictyozamites, Anomozamites.

Ginkgos

As has been mentioned earlier, there is one living member of this sub-group, Ginkgo biloba, which now appears to have come almost to the point of extinction in the wild but is now widely grown in parks and gardens. The most striking feature of the ginkgos are their curious leaves. These are strap-like and commonly show division into two or more segments. The segment lengths vary from one genus to another.

As distinct from many other gymnosperm sub-groups the ovules and pollen bearing organs were not borne in cones. The ovules occur singly or in pairs. Pollen-bearing organs are usually in the form of loose, catkin-like bundles.

This sub-group appears to have its origins as far back as 250 million years ago and they attained their greatest expansion about 120 million years later.

Fossil ancestral or related examples:
Trichopitys, Sphenobaiera, Baiera.

Other fossil examples: (some are only leaves or other fragments)
Eretmophyllum, Ginkgoites.