Adaptations shown by Parasitic Plants.

The Dodder

The dodder is a parasitic plant which attacks heather and gorse, and often docs much damage to clover crops.

The stem of the dodder appears as a fine pink thread coiling round the stem of a clover plant, to which it is fixed at frequent intervals by small suckers. From each sucker a strand of cells penetrates the stem of the host, and becomes connected with the vascular bundles. By this means the parasite is able to absorb water, salts and also more complex organic compounds from the clover plant. As the dodder has no roots, it is entirely dependent upon the host for its raw materials, and shows the reduction of structure that is usual among parasites. It has lost the chlorophyll present in green plants that are not parasites, and its leaves have become small and scale-like. In the late summer the dodder bears clusters of pink flowers, which occur in the axils of the scale leaves.

The seeds of the dodder require a fairly warm temperature for germination, and therefore do not develop until late summer. In this way it is ensured that there are clover plants sufficiently large to provide food for the parasite. The dodder seedling consists of a little yellow thread which stands erect at its tip and has a small root. The tip of the stem slowly grows forward, waving in a circle as it does so, and if a suitable host plant is touched the seedling coils tightly round the stem. Once it has reached a food supply the dodder sends suckers into the host, and the lower part of the seedling, which is in contact with the ground, shrivels and dies.

The Mistletoe

The mistletoe grows on trees, and in England is commonly found on the apple and poplar, and very occasionally on the oak. The mistletoe plant consists of a bundle of green twigs which branch dichotomously, and which bear pale green leaves arranged in pairs. The small flowers are produced at the end of the branches, and are followed by the large white berries. Inside each berry is a sticky pulp containing several seeds, which are dispersed by birds. When a bird attempts to eat a berry, the seeds stick to its beak owing to the sticky nature of the pulp. In order to clean its beak the bird wipes it on a twig, to which the seeds become stuck by the jelly. Germination takes place in the late spring, and the radicle grows straight into the branch, quite independently of the effect of gcotropism.

An adhesive sucker is formed which fixes the seedling firmly to the branch, and from which strands of tissue grow into the stem of the host. These strands tap the wood of the host, and absorb water and salts for the developing mistletoe shoot. Outgrowths are produced from the original sucker which grow along the branch of the host and from which fresh suckers are sent down into the wood. Each year the absorptive strands of the mistletoe penetrate deeper into the host to the extent of the width of one annual ring of the wood.

Since the mistletoe takes only water and mineral salts from its host, and has green leaves which function normally in building up organic food, it is said to be a partial or semi-parasite. Other examples of semi-parasites are the root parasites such as the yellow rattle and red rattle, whose roots tap those of other near-by plants by means of suckers.

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