Here it will be convenient to make some comments on the two major groups of seed-plants. Up until now we have met only those seed-plants which have no specially developed containers for the seeds to develop in. This group is called the gymnosperms, which means ‘naked seeds’. As has been suggested already, this is not a very good name for the group because in many cases (for example, conifers such as pines and cedars) the seeds are anything but ‘naked’. However, the fact remains that even in pine and cedar cones the protection of the seeds depends only on a number of cone scales tightly pressed together.
As a complete contrast to this situation the flowering plants, the angiosperms, always have definite containers for their seeds such as poppy capsules or pea pods. ‘Angiosperms’ means ‘seeds in containers’. The botanical name for the seed container in flowering plants is the ‘carpel’. It is, as it were, the hallmark of the flowering plants. Botanists studying fossil plants are therefore always on the lookout for reproductive structures bearing carpels when collecting fossil plants from rock exposures. This particularly so in older rocks because the discovery of carpels might pre-date all other known occurrences and could be very valuable in determining the evolution of flowering plants.
It will be understood, therefore, that when Caytonia [Fig.21] was first discovered in rocks about 160 million years old there was considerable interest among palaeobotanists. However, when examined in detail, this seed-bearing structure soon showed that from each seed a tube led to the outside. Therefore, the seeds were not in totally enclosed containers as they would be if the plant were truly angiospermic. Many years after the discovery of Caytonia it was shown in a number of species of Glossopteris that their seeds, too, were partially covered. When we come to discuss the angiosperms we shall see that a distinct advantage appears to be conferred on seed plants that have their seeds in some sort of container.
As yet there is no clear evolutionary pathway from any of the known fossil groups leading unequivocally to the angiosperms. More collection of fossil plant specimens continually takes place so that one day, no doubt, new discoveries will shed light on what is an intriguing mystery. As of now, the cycadeoids seem to be the most likely group to have given rise to the flowering plants.
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Among the fossil cycads and cycadeoids are also to be found specimens of another group which flourished from about 190 million years ago. These were the ginkgos, which were most numerous about 120 million years ago, but they went into a steep decline about 40 million years later as the flowering plants began successfully to invade the floras of the world. You will perhaps remember from Chapter 1 that this group has one surviving member to this day, thanks to its conservation in Chinese temple gardens.
This living representative, Ginkgo biloba, may grow to be a substantial tree. Its wood is very similar to that of Conifers such as pines and cedars. Its reproductive structures, though, are very different with its seeds borne singly or in pairs on separate stalks. Perhaps the most notable feature of the tree is the peculiar shape of its leaves [Fig. 22]. They resemble those of the Maidenhair Fern but are very much larger. Thus Ginkgo biloba is known as the Maidenhair Tree. Fossil leaves of ginkgos in the past showed many variations on the Maidenhair plan. Some had many lobes varying in length from one species to another and the bilobed ones also had species with very long lobes.
In this history of plants on Earth we are now approaching a time of important events which gradually brought about the situation we see today with the flowering plants being the most obvious ones because of the bright colours of their flowers. Although we cannot be absolutely sure and we must not make assumptions without evidence, it seems very likely that all plants earlier than the flowering ones were entirely in shades of green or perhaps yellow. This is due to the fact that these earlier plants almost always dispersed their pollen into the wind, a process that does not require coloured reproductive organs to attract animals. From this point onwards we shall see a gradual development of relationships between plants and insects (and other animals) so that pollen is transported by the insects to the female parts of the flowers. In their turn, plants started to produce brightly coloured structures to attract the insects.