One of the most important steps in the history of plants was undoubtedly the development of an enclosing mechanism for the seeds. We are so familiar with our own food plants such as peas and beans that we probably never stop to think how different our plant foods would be if the plants involved did not enclose their seeds in containers. For example, there would be no tomatoes, oranges, melons, cucumbers or marrows, to name only a few that we would lack. These are all cases where the seed containers themselves have become important sources of food. In many other cases it is the seeds themselves that are important. Another way of looking at plant foods is to try to think of any that are not produced by flowering plants. A glance at a typical cookery book shows that examples are almost non-existent. Except for mushrooms, almost all of our plant foods are produced by the flowering plants. This is not to say that it is always the seed containers themselves that are the important parts of the food but rather that the development of the containers conferred such advantages on the flowering plants as a group that they were able eventually to dominate the floras of the world.
Perhaps the most important food crops of the world are the so-called cereals. Even these seeds are in containers but because they are one-seeded containers the latter are not very evident. Nevertheless it is a fact that each wheat seed, for example, has two coats. The inner one is the seed coat and the outer is the container. However, because the two are fused together and inseparable it is not clear that two coverings are actually present. That a world without cereal crops is quite inconceivable is a measure of the debt that mankind owes to the flowering plants. Botanists have a special term for the seed container and its contained seed or seeds. They call it a fruit. This is not an unfamiliar word to us, but we usually think of a fruit as something sweet and which may, for example, be used as a dessert. We would find it difficult to ask the greengrocer for fruits such as marrows and cucumbers. However, botanists regard all seed containers and their seeds as fruits, whether they are sweet or not. This means that wheat and maize seeds are really single-seeded fruits.
However, in our history of plants it will be some time yet before the earliest flowering plants actually appear in the rocks as fossils. We are really setting the scene for when the moment arrives because once they are here the floras of the world will never be the same again.
Our present date is about 141 million years ago. It is the beginning of the Period that the geologists call the ‘Cretaceous’ because this was when the chalk of the white cliffs of Dover, so symbolic of England, was deposited. At this time there were coniferous forests extending to regions near the North and South Poles. Their fossil remains can be seen today in the Arctic tundra and in Antarctica where certainly no trees grow now. The world climate then was obviously much warmer than now, and it is possible that there was no ice at the Poles. Fossils also provide the evidence showing that the seasonless equatorial zone extended almost into what is now the Mediterranean area in the northern hemisphere and to the Cape of Good Hope in the south. Dinosaurs roamed far and wide over almost all the land areas of the world feeding on plants that reached a high degree of productivity on account of the very favourable climatic conditions. The Atlantic Ocean was gradually widening as the Americas moved steadily away from Europe and Africa.
It is very difficult to imagine the Earth without frost, snow, and ice. Commonplace human activities such as skiing and skating would only be possible with artificial surfaces. The very beautiful effects produced under low temperature conditions would be unknown. It is evident that the Earth is again on a warmer trend and may possibly become ice-free.
In the early part of the Cretaceous Period, before the advent of the flowering plants, the floras of the world were dominated by the non-flowering seed plants. The conifers, producing cones almost indistinguishable from those of the present day pines and cedars, were widespread from the North to the South Poles. As a result their fossil cones are to be found with virtually a world-wide distribution. Apart from the conifers, ginkgos were very common and so also were the cycads and cycadeoids. The latter became extinct about 85 million years ago but the ginkgos and cycads carried on to the present day with their numbers steadily declining. By contrast the conifers increased in numbers at this time, especially the pine-like ones (the Pinaceae). The clubmosses, horsetails and ferns played an ever-decreasing part in world floras.
The first appearances of the remains of the earliest flowering plants in the fossil record are sporadic in the extreme. Quite apart from the fact that they must have been very few in number and widely scattered, the haphazard way in which they became fossilized further complicates the problem of plotting their points of origin and early development. Here a few fossil leaves are found, there some pollen grains, and somewhere else flowers and fruits are discovered. After many years of collecting have now taken place, a survey of these early flowers reveals the fact that they can be put into three main groups. One has a strong resemblance to apple blossom, with the sepals, petals, and stamens in groups of five. Another is very similar to the spherical clusters that hang from plane trees and are such a familiar sight in London streets and squares in the autumn and winter. The third group has its flowers in catkins like those of hazel, willow, and poplar.
From these very early beginnings, in about 30 million years a huge number of flowering plant species evolved with the same rapidity with which trees appeared in the early colonization of the land. At the present day, flowering plant species are numbered in hundreds of thousands. Just one group alone, the orchids, number about 25,000 species. Flowering plants are undoubtedly the most successful ever to appear on Earth and, as we shall see later, were vital to the evolution of mankind and the subsequent dominance of humans over the entire globe.