Flowers

Types of inflorescence
The organs of sexual reproduction in the flowering plant division are flowers, and variation in their arrangement can be identified and named:

• spike is an individual, unstalked series of flowers on a single flower stalk, e.g. Verbascum;
• raceme consists of individual stalked flowers, the stalks all the same length again spaced out on a single undivided main flower stalk, e.g. foxglove (see Figure 7.3), hyacinth, lupin, wallflower;
• compound racemes have a number of simple racemes arranged in sequence on the flower stalk, e.g. grasses;

• corymb is similar to a raceme except that the flower stalks, although spaced out along the main stalk, are of different lengths so that the flowers are all at the same level, e.g. Achillea (see Figure 7.3). A very common sight in hedgerows;
• umbel has stalked flowers reaching the same height with the stalks seeming to start at the same point on the main stem, e.g. hogweed (see Figure 7.3);
• capitulum or composite flower forms a disc carrying flower parts radiating out from the centre, as if compressed from above, e.g. Inula (see Figure 7.3), daisy, chrysanthemum.

The number and arrangement of flower parts are the most important features for classification and are a primary feature in plant identification (see Classification and naming).


Flower structure
The flower structure is shown in Figure 7.4.

The flower is initially protected inside a flower bud by the calyx or ring of sepals, which are often green and can therefore photosynthesize. The development of the flower parts requires large energy expenditure by the plant, and therefore vegetative activities decrease. The corolla or ring of petals may be small and insignificant in wind-pollinated flowers, e.g. grasses, or large and colourful in insect-pollinated species. The colour and size of petals can be improved in cultivated plants by breeding, and may also involve the multiplication of the petals or petalody, when fewer male organs are produced.

The flower may include other parts:

• Tepals, where the outer layers of the flower have a similar appearance, making the sepals and petals indistinguishable. They are common in monocotyledons such as tulips (see Figure 7.5) and lilies.
• Androecium, the male organ, consists of a stamen which bears an anther that produces and discharges the pollen grains.
• Gynaecium, the female organ, is positioned in the centre of the flower and consists of an ovary containing one or more ovules (egg cells). The style leads from the ovary to a stigma at its top where pollen is captured.
• The flower parts are positioned on the receptacle, which is at the tip of the pedicel (flower stalk).
• Nectaries may develop on the receptacle, at the base of the petals; these have a secretory function, producing substances such as nectar which attract pollinating organisms.
• Associated with the flower head or inflorescence are leaf-like structures called bracts, which can sometimes assume the function of insect attraction, e.g. in Poinsettia.

The flowers of many species have both male and female organs (hermaphrodite), but some have separate male and female flowers (monoecious), e.g. Cucurbita, walnut, birch (Betula), whereas others produce male and female flowers on different plants (dioecious), e.g. holly, willows, Skimmia japonica and Ginkgo biloba.