The Excretion of Waste Materials in Plants

The relatively small quantities of waste products are prevented from interfering with the activity of a plant in two ways :—

They are converted into insoluble substances and retained in the cells.

They are transported to structures which either ultimately die, but which still serve a useful purpose to the plant; or are shed when their useful life is over.

The substances found in cells include granules or oil droplets. The granules may be crystals of calcium oxalate, glucosides or alkaloids, whilst the oil droplets present may give the plant its characteristic perfume. Their presence usually makes them unpalatable to animals, but whether they serve any other useful purpose is not known at present, so that it is probable that they are merely products of metabolism. As many of them are of commercial importance, the more common and more important ones are listed below. The alkaloids are of great value in medicine where minute quantities are extensively used as drugs.

Calcium oxalate in leaves of dog ’s mercury, enchanter ’s nightshade, rhubarb, horse-chestnut.

Glucosides—

Amygdalin in leaves of laurel, bitter almonds, cherry kernels. Digitalin in leaves of foxglove.

Alkaloids—

Aconitine in all parts of monkshood.

Atropine in all parts of deadly nightshade, henbane.

Cocaine in leaves of the coca plant of Bolivia and Peru.

Coniine in seeds of hemlock.

Morphine in young fruits of opium poppy.

Strychnine in seeds of nux vomica.

Oils and Oily Substances— Aniseed.

Cinnamon from bark of young twigs of the cinnamon tree. Cloves from flower buds of the clove tree. Eucalyptus.

Ginger from the rhizome of the ginger plant. Hop from female catkins of the hop. Lavender from flowers of the lavender bush. Pepper from berries of the pepper plant. Peppermint and menthol from the plant Mentha. Bcrgamot from a variety of orange. Camphor, a solid from the wood of the camphor tree. Turpentine, an oily substance from the trunks of pine tree, Resin, the solid residue from oil of turpentine. 2. The masses of dead cells which form the bark of trees commonly contain soluble substances known as tannins which are astringent to the taste—a property which renders the bark less likely to be eaten. Tannins present in oak bark are used for making ink. During the tanning of hides the protein substance, which easily decays, is converted into durable leather. Tannins are also present in tea-leaves. 2. Before the leaves and transient parts of flowers are shed, they commonly change to brilliant colours. Many of these are due to pigments termed anthocyanins—those coloured red, purple or blue being so because of the acid, neutral or alkaline nature of the cell contents surrounding the pigments respectively. Crystals of oxalate of calcium are also commonly found at this time.

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