The Plant Horticulture In Context

Bacteria are single-celled organisms sometimes arranged in chains or groups. They are not usually included in plant classification, but have great importance to horticulture by their beneficial activities in the soil, and as causitive organisms of plant diseases.Algae, comprising some 18 000 species, are true plants, since they use chlorophyll to photosynthesize (see Chapter 5). The division Chlorophyta (green algae) contains single-celled organisms which require water for reproduction and can present problems when blocking irrigation lines and clogging water tanks. Marine algal species in Phaeophyta (brown algae) and Rhodophyta (red algae) are multi- cellular, and have leaf-like structures. They include the seaweeds, which accumulate mineral nutrients, and are therefore a useful source of compound fertilizer as a liquid feed.

Fungi, or Mycophyta, are considered to be a division of the plant kingdom with many characteristics of plants, but they do not photosynthesize. They must, therefore, obtain their food directly from other living organisms, possibly causing disease (see Chapter 11), or from dead organic matter, so contributing to its breakdown in the soil. Lichens form the division Lichenes, but their classification is complex since each lichen consists of both fungal and algal parts. Both organisms are mutually beneficial or symbiotic. The significance of lichens to horticulture is not great. Of the 15 000 species, one species is considered a food delicacy in Japan. However, lichens growing on tree bark or walls are very sensitive to atmospheric pollution, particularly to the sulphur dioxide content of the air. Different lichen species can withstand varying levels of sulphur dioxide, and a survey of lichen species can be used to indicate levels of atmospheric pollution in a particular area. Lichens are also used as a natural dye, and can form an important part of the diet of some deer.

Mosses and liverworts. The 25 000 species are included in the division Bryophyta, and have distinctive vegetative and sexual reproductive structures, the latter producing spores which require damp conditions for survival. The low spreadingcarpets of vegetation present a weed problem on the surface of compost in container-grown plants, on capillary benches, and around glazing bars on greenhouse roofs.Ferns and horsetails, in the division Pteridophyta, represent 15 000 species which have identifiable leaf, stem and root organs, but produce spores from the sexual reproduction process. Many species of ferns, e.g. maidenhair fern (Adiantum cuniatum), and some tropical horsetails, are grown for decorative purposes, but the common horsetail (Equiseturn arvense), and bracken (Pteris aquilina) which spread by underground rhizomes, are difficult weeds to control.

Seed-producing plants (Spermatophyta) contain the most highly evolved and structurally complex plants. There are species adapted to most habitats and extremes of environment. Sexual reproduction produces a seed which is a small, embryo plant contained within a protective layer.The class Gymnospermae has approximately 700 surviving species which produce 'naked' seeds, usually in cones, the female organ. This class shows some primitive features, and often displays structural adaptations to reduce water loss (see Figure 4.2). The order Ginkgoales is represented by a single surviving species, the maidenhair tree (Ginkgo biloba), which has an unusual slit-leaf shape, and yellow colour in autumn. The most important order to horticulture, the conifers (Coniferales), provides many families with horticulturally interesting plant habits, and foliage shape and colors. The Cupressaceae, for example, include fast-growing species which can be used as windbreaks, and small, slow-growing types very useful for rock gardens. Taxaceae, a highly poisonous family, contains the common yew (Taxus baccata) used in ornamental hedges and mazes.