If you understand a little about the nature of plants, you can enjoy them more, look after their nutrition, and control pests and weeds, with more confidence.

Plant nutrition

The essential difference between plants and animals is that plants are able to make their own food from raw materials, while animals can only eat either plants or other animals.

In making their food, plants use carbon dioxide, water and sunlight to make sugars, by the process called photosynthesis. Sun­light is trapped by the chlorophyll in the leaves – the substance that makes leaves green – and its energy is used in combining water and carbon dioxide; during that process the oxygen which we ourselves need for breathing is released. The carbon dioxide required is extracted by the leaves from the small quantities always present in the atmo­sphere. Water is drawn up from the soil by the roots, although small quantities can also enter through the leaves.

The sugars made in the leaves are con­verted into other organic products using nutrients extracted from the soil.The sugars and other products are moved around through the plant to provide food for growth, for storage, or for reproduction. The nutrients and water are carried up to the leaves from the roots in the sap; nearly all of the water extracted from the soil is used not in photosynthesis but to keep the leaves rigid and is lost in transpiration.

For this essential first stage of sugar manu­facturing to work, several conditions must be satisfied. The leaves must be able to obtain carbon dioxide from the atmosphere, which means that the breathing pores or stomata in them must be open; very windy weather or a shortage of water in the plant will result in them being shut to conserve moisture and thereby prevent photosynthesis. The leaves must be exposed to sunlight, as photosyn­thesis cannot take place in the dark. Also nutrients must be available, both for the manufacture of the next stage (otherwise the concentration of sugars will prove lethal) and the previous one so that chlorophyll and other enzymes are available.

Flowers

 The ‘perfect’ flower consist of sepals, petals, ovaries, stigmas, styles, stamens with anthers and filaments, and nectaries. Its function is to enable the fertilization of egg cells by the pollen, so leading to the produc­tion of seeds to make the next generation. Many flowers are not ‘perfect’ and one or more of the above parts may be missing.

The sepals are often rather green and leafy and are mainly involved in protecting the flower buds. In a few species they become highly colored and act like petals, eg in clematis. The petals are usually the attractive parts of the flowers, giving the floral display. The ovaries are where the seeds are devel­oped and the stamens where the pollen is manufactured and shed.

Most flowers are designed for pollination by various animals, including birds, but some plants are wind pollinated, e.g. hazel, and these have no need for showy petals. In wind-pollinated plants, the flowers tend to be placed where the wind will catch them, or to open before the leaves. To induce insects to visit, many flowers secrete sugar-rich nectar. Fragrance is also used to attract pollinators.

In many plants, the flowers are imperfect, that is only one sex is present. This is a device to ensure cross-pollination and avoid self-breeding; if these plants are grown for their fruit, both sexes must be present. Hollies are an example.

An alternative strategy for ensuring cross-pollination is for the plant’s own pollen to be ineffective at fertilizing the flowers. Here the flowers must be fertilized by pollen from a different plant of the same species if fruit and seed are to be formed. Many fruit trees, especially apples, have this mechanism and will only crop if two or more compatible trees are growing nearby.

Many garden plants are selected with abnormal flowers, such as extra petals. Some of these abnormalities occur naturally in certain groups of plants. For instance, a number of clematis have extra petal-like structures which are modified stamens and are called staminoides.

In most plants, flowers are only produced by specimens which are growing satisfactorily. The incitement to produce flowers often follows on a build-up in the amount of sugars in the top of the plant. In fruit trees, this can be artificially stimulated by partially girdling the stem, thereby restricting the passage of sugars down to the roots. Plants will not flower well if grown in more shade than the species likes, although other plants will not tolerate full sun. A severe pruning will cause the plant to concentrate on vegeta­tive growth at the expense of flower produc­tion.