The science of life : fully illustrated in tone and line and including many diagrams

BOOK 4

them at our pleasure. And so with Protozoa; provided they are big enough to chop in pieces, and provided there is a bit of nucleus in every piece, we can force reproduction on them at the edge of the knife.

In plants, not only are such methods possible, but they have been used for practical ends time out of mind. A mere post of green willow wood may take root and sprout and turn into a new willow bush, and a willow twig will even produce leaves and roots when suspended in damp air. Gardeners reproduce a chosen plant by taking slips

lag see Fig. 162. another variety.

Left: Insertion into a cut in the stock. Centre: The graftis tied tight. Right: The region, the graft is cemented with grafting-wax.

or cuttings—shoots bearing one or a few buds—and simply planting them in soil. If undue evaporation is prevented aboveground, and the parts below-ground are warmed a little to expedite root-production, many plants can be successfully propagated in this way.

Plants which happen not to grow well from cuttings may often be propagated by layering. A branch is bent down and part of it lightly buried in soil ; when it forms roots it is cut through near its attachment to the parent stem, and grows into—or indeed is already—a new plant.

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THE S@IENCGE OF LIFE

Two twigs of one variety of tree being grafted on to a stock of

G@HAPT ER 2

Or the method of grafting may be employed. Here the detached bit of living tissue is not planted in the soil, but is made to unite with the tissues of another plant, the host or stock. In grafting, the whole plant is not reproduced, since the graft is served by the roots of the stock and never forms any of its own ; but it grows and forms its own leaves and flowers and fruit. Grafting ensures a real though partial reproduction. Sometimes only a single bud is grafted, as with roses or peaches, and is then inserted under the bark at the side; or whole stems are made to unite end-to-end at their cut surfaces. In many animals, artificial propagation could be carried out on a similar scale; but they are not usually of any commercial value. The only exceptions are bath-sponges. These can be cut into little pieces, each of which will grow into a new sponge; and experiments are being conducted to see whether this method can be made commercially useful.

But the most extraordinary method of artificial reproduction is that which we have already mentioned in Book 2—reproduction not by any procedure so mild as cutting or grafting, but by the really drastic method of rubbing the animal through a fine-meshed sieve of finest bolting-silk until it is totally disintegrated into single cells and small cell-groups. The mere cellsediment which is first produced begins to organize itself into separate balls and lumps. Finally, each lump, provided it is neither too large nor too minute, will organize itself into a miniature animal once more. Before this power the fable of the Hydra pales; an organism which can have every vestige of its organization destroyed and whose scattered débris can