There are thought to be about 300,000 species of flowering plants. Attempting to produce an ordered classification of such an enormous group of species is a monumental task. Features such as the numbers of sepals and petals in the flower, whether or not these flower parts are separate or fused together, whether or not the individual flowers are so tightly packed together that the result looks like one flower (as in a daisy), are all used to arrange species into sub-groups. Vegetative features, that is to say details of the plant’s stem, leaves and roots, are rarely used because these may vary enormously among a sub-group having more or less identical flowers. This means that a species may develop a very strange outward appearance, perhaps to cope with a difficult environment, and yet produce a flower that is typical of its sub-group.
The flowering plants can at least be divided fairly easily into two large sub-groups. These are known as the monocotyledons and the dicotyledons. A cotyledon is a seed-leaf and, depending on the mode of germination, is the first structure to appear above the soil. In the case of mustard seed, when this germinates there are two cotyledons which come out of the soil indicating that the mustard plant is dicotyledonous. Onion, a monocotyledon, has only one cotyledon that appears out of the seed. We will now deal with the general features of the two groups.
This is by far the larger of the two sub-group. Its salient features are as follows:
a) The seeds typically have two cotyledons.
b) The root system usually has the form of a tap root with side branches, not merely a mass of fibrous roots.
c) The form of the plants is often a tall woody stem with many side branches.
d) In the early stages, the conducting tissue in the stem is a number of strands arranged in a ring. Later these may become fused together in a woody cylinder that is added to year by year. In this way ‘tree rings’ may form.
e) The leaves are usually in two parts: a blade-like portion, the lamina, and a stalk, the petiole. The veins in the leaves often form a complex network.
f) The flower parts are usually in fours or fives although, inevitably in such a large sub-group, there are numerous exceptions to this.
g) The outer parts of the flower, the sepals are usually of a different appearance from the petals.
Ranunculus (Buttercup), Bellis (Daisy), Taraxacum (Dandelion), Quercus (Oak), Cheiranthus (Wallflower).
This is the smaller sub-group, but it is nevertheless very important since it contains all of the cereal crops. Its main features are:
a) The seeds have a single cotyledon.
b) The root system usually consists of a mass of fibrous rootlets.
c) Main trunk systems such as are seen in palms and yuccas are rare elsewhere in the sub-group. Many monocotyledonous plants have many small unbranched stems.
d) The conducting tissue in the stems is scattered in a random fashion and only rarely becomes involved in trunk wood formation.
e) The leaves are often long and strap-like’ at their bases they may be wrapped around the stem in a sheathing fashion. Long, parallel veins run up the length of the leaves.
f) The flower parts are often in threes.
g) The outer parts of the flower may not obviously separable into sepals and petals, as in tulips and crocuses for example.
Triticum (Wheat), Lilium (Lily), Convallaria (Lily of the Valley), Scilla (Bluebell), Musa (Banana).
Towards the end of the Cretaceous Period, about 65 million years ago, the flowering plants were widespread in the world’s floras. The conifers were still important but definitely under pressure from the flowering plants. The other plant groups continued their steady decline.