From this time onwards there was a gradual change in the Earth’s flora. Although some clubmosses, horsetails and ferns persisted until the present day, they never again achieved the level of importance they had in Coal Measure times. The onset of their decline coincided with the increase in the numbers of seed plants. At this time, too, we get a clear indication from fossil plants of considerable changes in the Earth’s climate. The continents had now merged together to form one large landmass, which geologists call Pangaea. A considerable part of which, those areas which are now Antarctica and the southernmost parts of Australia, South Africa, and South America, were in the vicinity of the South Pole and in fact show evidence of a widespread Ice Age. As a complete contrast there is also evidence in the form of the fossilized trunks and stumps of trees that forests grew either during interglacial periods or after the Ice Age quite close to the South Pole.
What we can see from this is that at one time the South Polar area was covered in thick layers of ice whilst at other times, the climate was temperate enough for forests to be able to grow. We are now all getting used to the idea of climatic change due to the Greenhouse Effect. Certainly in past geological times there have been major global changes in climate and there is no good reason to suppose that there will not be further ones in future.
New forms of early conifers appeared [Fig. 18], still without actual cones but with foliage very like that of the cypress trees we see today. The seeds were borne singly on individual stalks; it would be some time yet before true cones appeared as fossils. Another newcomer at this time was Glossopteris, a rather unusual tree [Fig. 19]. Scott’s party, returning back to base from the South Pole on the ill-fated 1912 Expedition to the Antarctic, carried on their sledge some Glossopteris leaves. The name refers to their tongue-like shape. Fossil collection over the southern hemisphere part of Pangaea has revealed the fact that this unusual tree had a very widespread distribution. Its roots were also highly characteristic in having peculiar cavities within them.
Glossopteris trees flourished from about 280 million years ago until about 200 million. As they reached their peak, in the middle of this period, another group of seed-plants gradually emerged. These were the cycads. It is somewhat difficult to describe members of this group because their living representatives are not often seen by persons in countries outside the tropics. They are cultivated in greenhouses in Botanic Gardens, but they cannot be grown in parks or gardens away from the tropics unless the area has a local climate that happens to be unusually mild and not typical of the region as a whole.
The cycads as a group tend to be somewhat dissimilar from one another and Cycas itself, after which the group takes its name, although the largest is not typical of all of them. Superficially Cycas looks rather like a palm but when studied in detail it is seen to be very different from the palms in a number of ways. The wood and leaf structures are not the same as in palms and the way in which the seeds are borne is fundamentally different. Except in Cycas, the seeds are usually set deeply in cone-like structures so that they are not easily accessible to animals who might want them as food.
Cycads are very common as fossils as are a very similar group called the cycadeoids. Whereas the cycads have living representatives, the cycadeoids became extinct about 80 million years ago, about 120 million years after their first appearance. Although they left no direct descendants, the group is of great interest in being possible ancestors of the flowering plants through many devious connections. The reproductive organs in cycadeoids such as Cycadeoidea [Fig. 20], as seen on the front cover, certainly give the impression of similarity to a flower. Indeed, when they were first discovered they were thought to be true flowers, but detailed study showed that their seeds were borne in a central cone-like structure as is also the case in most cycads. Also, it is now thought unlikely that the structure ever opened like a flower but instead pollen was shed internally on to the ovules.