Chapter 4

Non Plant Kingdoms

Animals

The animal kingdom includes a very large number of species that have a significant influence on horticulture, mainly as pests (see Chapter 18) or as contributors to the recycling of organic matter (see Chapters 3 and 13).

Some of the most familiar vertebrate animals include mammals, birds, fish, reptiles and amphibians. Mammalian pest species include moles (see p. 227), rabbits (see p. 226), deer, rats and mice. Bird pest species are numerous, including pigeons and bullfinches, but there are very many that are beneficial in that they feed on harmful organisms – for example, tits that eat greenfly. Less familiar are the nematodes or roundworms which are worm-like invertebrates and include a very large number of plant disease-causing organisms including stem and bulb eelworm (see p. 248), root knot eelworm (see p. 249), chrysanthemum eelworm (see p. 248) and potato root eelworm. The arthropods are the most numerous animals on earth and include insects, centipedes, millipedes and spiders; many of these are dealt with in the chapter on plant pests (Chapter 18), but it should be noted that there are many that are beneficial – for example, honey bees and centipedes, which are carnivorous and many live on insect species that are harmful. The segmented worms includes earthworms, which are generally considered to be useful organisms, especially when they are helping to decompose organic matter (see p. 159) or when improving soil structure (see p. 147 ), but some species cause problems in fine turf when they produce worm casts. The molluscs are best known for containing the major pests slugs and snails (see p. 229).

Bacteria

Bacteria are single-celled organisms sometimes arranged in long chains or groups. They are autotrophic. Some photosynthesize but others are able to make organic molecules using the energy released from chemical reactions usually involving simple inorganic compounds. They have great importance to horticulture by their beneficial activities in the soil (see p. 159) and as causative organisms of plant diseases (see p.262).

Algae and lichens

The algae, comprising some 18,000 species, use chlorophyll to photosynthesize (see Chapter 9). The green algae group includes single-celled and multicellular organisms from which land-living plants are thought to have evolved. Some can present problems when blocking irrigation lines and clogging water tanks. Blanket weed is a filamentous green alga which is a nuisance in freshwater ponds. Green algae also include some marine seaweeds. Other marine algal species include brown algae and red algae which are multicellular and have leaf-like structures. They are the familiar seaweeds, which accumulate mineral nutrients and are therefore a useful source of compound fertilizer as a liquid feed. (The ‘blue–green algae’, which can cause problems in water because they produce unsightly blooms but are also toxic, are actually cyanobacteria and are placed in the kingdom Prokaryota.)

Lichen

(Figure 4.14) 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. Many contribute to the weathering of rock in the initial stages of soil formation (see p. 142). Lichens are also used as a natural dye and can form an important part of the diet of some deer.

Viruses

Viruses are not included in any of the kingdoms. They are visible only under an electron microscope and do not have a cellular structure, but consist of nucleic acid (DNA or RNA) surrounded by an outer protein coat. They do not have the cytoplasm, organelles and internal membranes found in the cells of living organisms (see Chapter 6). They cannot only grow, move or reproduce within a host cell so they are not included in the classification of living things. Viruses survive by invading the cells of other organisms, modifying their behaviour and often causing disease – for example, arabis mosaic, chrysanthemum stunt, cucumber mosaic, leaf mosaic, plum pox, reversion, tomato mosaic and tulip break (see p. 264).