This section is from the book "Commercial Gardening Vol1", by John Weathers (the Editor). Also available from Amazon: Commercial Gardening, A Practical & Scientific Treatise For Market Gardeners.
Amongst flowering plants many species, including the Broomrape (Orobanche) that lives attached to the roots of Clover, and the Dodders (Cuscuta) that live on Clover, Nettles, Hop, and other wild plants, have no chlorophyll in their tissues, and cannot manufacture their own food. They must needs attach themselves to the roots or stems of certain green plants and absorb their food in an organized form, thus robbing and injuring their hosts to a greater or less extent. The Broomrape sometimes attaches itself to the roots of Pelargoniums in pots, and one of them, kept under observation by the writer, was allowed to flower, The result was that the Pelargonium was stunted in growth and failed to recover itself, even after the parasite was removed. Such plants are termed parasites, because they absorb their food from living plants. The great group of fungi have no chlorophyll in their tissues, and many of them are parasites, like the Club-root fungus of Cabbages, the Mildew and Rust of Roses and Chrysanthemums, the Rust and Brand of Wheat, and many other cultivated plants. A large number of them are very minute, one-celled, and capable of producing diseases in plants, man, and other animals. In these two latter cases they owe their existence indirectly to green plants as the first and only manufacturers of organic food. On the other hand, many fungi are harmless, because they live upon dead and decaying plants and animals, and are termed saprophytes. The Mushroom is one of them, and lives upon fermenting manures and other decaying matter. So far it is the only plant without chlorophyll, that is of any importance to cultivators in this country. All fertile soils swarm with minute, one-celled fungi or microbes, living upon dead matter and converting much of it into a soluble form, suitable as food for green plants. In a word, they are the agents alike of decay and fertility, preparing the soil, the manure, and leaf heaps for the use of the higher plants.
Lichens are composite plants, consisting of a fungus and a small green alga, working in co-partnership for their mutual benefit. Even some of the higher plants, including several forest trees, have messmates or co-operators amongst fungi, large enough to be seen by the naked eye (fig. 10). The fungi closely invest the fibrous ends of the roots and absorb the food they require from the trees. On the other hand, some of the waste products of the fungi are required by the trees to complete their bill of fare. It is not yet determined to what extent this co-operation prevails among cultivated plants, but some plants difficult to cultivate may really require this kind of assistance. It is well known that Rhododendrons and other plants belonging to the same family like a peaty soil and hate lime in any form. In all probability the lime destroys the microbes in the peat that are essential to the welfare of the Rhododendrons.