TREES THAT GO OFF WITH A BANG

Evening Star, Issue 20373, 3 January 1930, Page 10

TREES THAT GO OFF WITH A BANG

HOW MATURE SOLVES THE POPULATION PROBLEM

[By Professor J. A hthur /Thomson, m ‘John o’ London’s Weekly.’J

In late autumn many of the fields in the North of Scotland are still covered with “stocks”—the harvest has not yet been stacked. But a walk in the country shows us that Nature has already sown most of the seed for next year’s sprouting, and it is interesting to study the methods of her sowing, which arc very diverse.

A seed is, of course, a young plant which has been developing for a while in intimate union with its parent, but has now passed into a quiescent state. For many hundreds of millions of years after plants began to cover the earth there were none with seeds. The mosses, ferns,, horsetails, and the like are seedless, multiplying mostly by snores or single germ cells. It was not till the time of the Upper Devonian that seed plants appeared, and it was one of the great steps in organic evolution. What then began to be liberated from the parent were no longer single cells, .but young plaints; and the contrast between spore multiplication and seed multiplication is a little like that between a mother salmon that liberates ego-s in the gravelly bed of the stream an 3 a mother otter which gives birth to a well-formed cub. THE NEW GENERATION. Having seeds means that what aro set adrift from the parent to start a new generation are well-equipped young plants, with a better chance of survival than single germ cells can ever have. But the drawback is that seeds, being enormously larger and heavier than single germ cells, are not so readily scattered by the wind, and _it is very interesting to inquire how this difficulty of dispersal has been solved. The fruit is the ripe seed bos or ovary, often with accessory parts, while the seeds are embryo plants inside the fruit. But while this is a clear enough distinction, it is often of little importance, since the fruit and the seed are almost identical in cases like thistledown or grains of wheat or small nutlets. Tiie wind-borne fruit of the dandelion consists of a delicate filament with a nutlet at the base and a beautiful radiating parachute at the top; and inside the nutlet is a single seed. What is sown in this case is iruit, and this is the simplest kind' of sowing. Similarly for the fruits of grasses and for all sorts of dry nutlets. THE SEEDS’ DEBUT.

When the time for sprouting comes the seedling has to burst out of the seed coats and then through the decaying wall of the fruit. The same is true of the dry fruits called “splitters” or schizocarps, as in hemlocks and labiates, where the fruit divides into several pieces without allowing the seed to escape. What is sown in such cases is a piece of fruit. Entirely different, if one thinks clearly, are the dry box fruits which open to some extent and allow the seeds to tumble out. They aro what the botanist calls dehiscent, whereas those we have previously spoken of' are indeiiiscent. The simplest example of a box fruit or capsule is a pea pod, a single transformed leaf structure (car;sl) folded on itself. Much more complicated is a poppy head, where the seeds tumble out through little holes in a ring at the top. On a sunny day in autumn wo hoar the pods of the broom exploding, jerking the seeds out for some feet;, the same may be seen in, pansies and lupins, and in the balsam with its :p----propriate name “ Touch-nm-mt ” < Impatiens noli-me-tangere), where a slight touch at the p • .t.i.u . about u violent dehiscence and dispersal. The climax is in the sand-box tree (Hnra Crepitans), where the drying of the carpels that make the seedbox causes an explosion, with a sound like a pistol shot, and the seeds aro thrown out to a considerable distance. SOME CURIOSITIES.

This is sensational, but far more interesting is the fact that the bursting of the walls of the seed-box corresponds to the autumnal, wrinkling'and withering and fall of ordinary loliago leaves. What science does is to bring curiosities into line. As might bo expected, the explosive method of sowing is practically confined to hard or dry fruits, but we should not forget tho Californian inisteltoe, whoso soft fruits burst with such violence that the glutinous seeds may be thrown against branches several yards off. This is in striking contrast to, the quiet, way in which the ,European mistletoe has its seed planted on the apple branch by the bill of the missel thrush. The case of the Californian mistletoe links the explosive fruits to those that are swallowed, as is usual with soft fruits, whether of tho “stone’’ or Mio berry typo. In the stone fruits, such as cherry and plum, the seeil lies inside the hard innermost layer of the fruit: it is the familiar “kernel” inside tho “ stone.” In true berries, on the ether hapd, such as gooseberries and grapes (and oranges, 'strange t a say), the hard-walled seeds lie embedded: in the juicy pulp. In cither case, the hard wall, which belongs to tho fruit in a cherry and to the seed in a grape, usually saves the_ delicate seed from being digested during its passage down the bird’s food canal. FRUIT-EATII.'G BIRDS.

It should ..is noticed, however, that fruit-eating birds usually eject largo stones, swallowing only the pulp. As far as dispersal is concerned, berry fruits are probably more successful than a* o fruits; the uninjured small seeds may be voided many miles from tbo place whore they were swallowed. Many water plants, such as pond weeds, have their seeds scattered by water currents, but Darwin showed that some may be carried jn the mud on birds’ feet from one river to another. Tbo same will happen with the seeds of land -plants if they get involved in the mud balls formed on a bii ’’s shanks or toes. Not a few of tbo imoponing fruits have burs and hooks on their surface, and thus become attached to passing animals, from which they eventualy fall off. This is well illustrated by burdock, bound’s tongue, medick, and Jack-nn-tlie-hedgo or cleavers.

fn ni"; t b ol’-’-P /• important that the seeds should bo scattered than that they should bo scattered far; thus the so-called winged fruits of maples and the like Fall gyrating through the aii and get beyond the shaded region around the parent tree. Yet there are many subtleties which forbid us from thinking that any odd scattering will servo. Thus the seeds cf the broomrapo will not sprout unless they are borne into actual contact with Iho roots of ;ho broom or the like on which the weird plant becomes parasitic; and there are many orchid seeds—numerous, minute, and wind-borna—■•which cannot germinate without the co-operatioli 01. a particular partner fungus.