SEED-BEARING PLANTS

THE true seed-bearing plants are all comprised in the two divisions Gymnosperma: and Angiospermaj, to which we must now direct our attention. Together, these two groups are often referred to as ‘Flowering Plants.’ In botanical language, a ‘flower ‘is a special kind of aerial shoot serving ultimately for the production of seeds. The part of the stem on which the flower is borne is known as the ‘receptacle,’ while the various parts of the flower itself are regarded as modified leaves. In such a flower as a buttercup, the ‘male ‘and ‘female ‘organs are called ‘stamens ‘and ‘carpels ‘

respectively. A carpel may be thought of as a leaf folded upon itself and joined at the edges, thus forming a chamber or vessel enclosing the egg-cell, or egg-cells, as the case may be. Its upper part, which is frequently a sticky knob, is the ‘stigma ‘to which pollen-grains adhere, and from which, after germinating, they grow downwards as ‘pollen-tubes ‘into the egg-chamber to fertilise its contents. The grouped carpels of a single flower, especially if they are closely united, are often spoken of as its ‘pistil.’ Each of the male organs, or stamens, consists typically of two parts : (1) a stalk or filament which bears aloft, (2) paired pollen-sacs, or ‘anthers.’

Outside these two groups of ‘essential organs ‘there is commonly an arrangement of ‘petals ‘, often gaily coloured and attractive to insects which render an important service as pollen-conveyers when they fly from one blossom to another. Together, the petals constitute the ‘corolla,’ within which, in many kinds of flowers, may be found honey-secreting glands, or ‘nectaries,’ whose presence makes the visits of insects ‘worth while,’ even when they can make no use of

the pollen itself as food. Finally, the outermost whorl of floral leaves, called ‘sepals,’ serve as the ‘calyx,’ whose normal function is to protect all the other members of the shoot while they are still folded together and in process of development as a ‘bud.’

‘CONES’ THAT ARE REALLY ‘FLOWERS’

IN flowering plants an alternation of spore-bearing and sexual generations can still be traced, but only by means of the microscope; for the whole process is disguised, so to speak, under the apparently straightforward methods of pollination and seed-production. For our present purpose we need only note that the vegetative, leaf-bearing body of a flowering plant represents the spore-producing generation, of which the Fern as such was our outstanding example. It is the sexual generation, typified by the prothallus from which the Fern took its origin, that has disappeared from view.

Not all flowers have the elaborate structure described above. Indeed, the name ‘Gymnosperma? ‘refers to the fact that in members of this division the seeds are ‘naked : ‘I.e. the seed-producing leaf, or carpel, is not folded together to form a chamber or ‘pod,’ but bears the essential organs openly on its concave inner face. The young cones of Pines and Larches are really collections of such simple flowers; and if we tear apart some of the overlapping scales, we shall see the carpels with their immature seeds or ‘ovules.’ Pollen, which is produced in great abundance in the anthers of small stamens arranged in dense clusters on neighbouring shoots, is blown by the wind in dry weather into the crevices between the cone-scales, and in this way fertilisation is effected.

Even in the great Angiosperm division, which—coniferous trees apart—comprises nearly all the conspicuous plants of our countryside, the flower is often very simple. Take, for instance, the Goat Willow—widely known as ‘Palm.’ Here, the female flower consists of a single carpel mounted on a tiny bract-like leaf in association with a little nectary or ‘honey-pot; ‘the male flower is just a pair of stamens, also with a honey-pot for the delectation of insect visitors. The two kinds of flowers are massed together in groups— often called ‘catkins,’ and are borne on separate trees, so that ‘self-pollination ‘is out of the question. But bees are attracted by the scent of the nectar, and keep up a constant

traffic from one tree to another, with the result that the female Goat Willows never fail to mature large crops of fluffy seeds.

THE BARGAIN BETWEEN INSECT AND FLOWER MANY trees and plants whose flowers are inconspicuous and lack nectar—e.g. the Hazel, the Oak and the Stinging Nettle—must rely upon the wind to secure the transfer of pollen from the anthers of their stamens to the stigmas of their carpels. But in the case of showy and Saveet-scented flowers we may conclude that insects of one sort or another act as the emissaries of Cupid. Insect-fertilised flowers produce less pollen than those which rely upon the wind as their carrying agent, while the individual grains, instead of being dry and dust-like, are usually adhesive, so that they attach themselves readily to an insect’s proboscis, or to the hairs of its body and legs. The arrangement of a given flower’s parts will naturally be found to correspond with the structure and habits of the particular kinds of insect whose visits are specially catered for. Hence, wrc find a great variety of floral mechanisms, all directed to this one end of pollination, with its sequel, the production of seeds.

Botanists recognise two chief subdivisions of Angiosperm plants, in one of which the embryo starts with a single seed-leaf, while in the other it has two. The first subdivision includes the grasses, cereals, rushes, lilies, irises, orchids, and so forth—plants which in general have radiating or parallel leaf-veins, and the parts of whose flowers are nearly always arranged in threes. In the other subdivision, which, excepting the cone-bearers, comprises practically all the trees, shrubs and herbs of the whole world, the leaves are net-veined, while the floral parts are commonly grouped either in fours or fives.

A few examples will help us to grasp how closely the fertility of a flower is bound up with its structure and mechanism. We should bear in mind that in each instance ‘cross-pollination ‘—I.e. the transfer of pollen from one bloom to another —is the chief end in view : ‘self-pollination,’ when the pistil of a flower is dusted with the product of its own anthers, usually eventuates only as a last resort. Suppose we take as our types the following : Perennial Rye Grass, Yellow Iris, Purple Orchis, Wild Arum, Dog Rose, Bird’s-foot Trefoil, Primrose, Foxglove, Dead Nettle, Hogweed and Daisy.

THE INDEPENDENCE OF THE ‘TINKER-TAILOR’ GRASS OF this varied selection, only the Rye Grass can dispense with the service of insects if cross-pollination is to be achieved. It is a common, wayside species, and may be recognised by its flattened ‘spikelets,’ from eight to twenty of which are carried by each main stem. Children call it ‘Tinker-Tailor Grass.’ Each spikelet contains eight or ten simple flowers, which are completely hidden from view until the time of blossoming, which is usually on a warm, sunny day.

There are three stamens with large anthers which dangle freely from the spikelet on delicate filaments. The pistil, which stands in the centre of the flower, consists of a pear-shaped ovary, bearing a pair of delicate, feathery stigmas at its upper end, while at its base there are two small processes known as ‘lodicules.’ The latter, by swelling up when the time for blossoming arrives, force apart the chaffy bracts which envelop the flower, and thus allow the stamens and stigmas to expand. These details can easily be made out by using a pocket-lens.

The dry, powdery pollen which escapes from the anthers is caught up by the passing breeze and carried hither and thither. Much of it is wasted; but some reaches the plumelike stigmas of other Rye Grass flowers, thus effecting their cross-pollination and enabling them to set good seed.

SWEET GIFTS WITH A PURPOSE: THE IRIS AND THE HUMBLE-BEE Now compare these floral arrangements, common to most grasses, with those of the Yellow Iris or Flag, which depends mainly on humble-bees as its pollen-carriers. Two sets of petal-like leaves can be distinguished, the three outer ones being large and down-curved, the three inner narrow and erect. All six are joined together at their bases to form a short tubular region whose inner surface secretes nectar. If an actual flower is examined, the Iris is seen to offer three separate lines of approach to its store of sweetness, to which the bee gains access by forcing its head and shoulders into a kind of tunnel, roofed over by a pair of essential organs. In doing this, the insect’s back comes into contact first with the stigma, then with the anthers of the stamen. Assuming that it has come from a nearby Iris, it will cross-pollinate the one which now engages its attention; and from this, in its

turn, pollen is likely to be carried by the industrious insect to the next flower on its visiting list.

If you watch insects, especially bees, at work in a garden on a sunny morning, you will notice that they have formed the habit of flying rapidly from one bloom to another of the same kind. A humble-bee, for example, which has been busy with the snapdragons does not break off suddenly to pay calls on the delphiniums. Colour seems to be their chief guide. This is fortunate from the standpoint of the flowers, whose egg-cells need to be fertilised by pollen matured in the anthers of their own species. Alien pollen has no power to set in motion the delicately adjusted machinery which results in seed-production.

If the adaptation of the Iris to the requirements of its bee-visitors is remarkable, that of the Early Purple Orchis is even more so. Here, the various parts of the flower are so much modified and twisted as to be hardly recognisable at first glance. Of the six petal-like leaves, no less than five are grouped together to form a kind of hood over the essential organs, while the sixth is expanded below into a sort of lip, or ‘labellum,’ which serves as an alighting place for insects. This labellum also extends backwards as a hollow spur, within which the nectar is secreted and stored. When we discover that the stigmatic lobes, connected with the ovary, are located just inside the opening of this spur, we realise that the bee cannot help touching them when it comes to the flower to gather honey.

To make assurance doubly sure, the Orchis also dispenses its pollen in a highly novel and ingenious manner. We may say that it has reduced its stamens to one, while the pollen is made up into two convenient masses, connected by tapering stalks with round, sticky discs. The latter lie loosely in a cup-shaped container, called the ‘rostellum,’ placed just in front of, and a little above, the stigmatic lobes. You will grasp these details most readily if you open your mouth before a mirror and examine the entrance to your own throat. The tonsils on either side represent the stigmatic lobes of the flower, while the pendent uvula in the middle line takes the place of the rostellum and its pollen masses.

When a bee alights upon the labellum, and inserts its proboscis into the throat of the flower in search of nectar, its head comes into contact with the rostellum, and it flics away with the sticky discs of the pollinia adhering to its forehead,

like a pair of horns. As the bee travels through the air, the discs contract unequally in drying, with the result that the pollinia bend forward and at the same time diverge slightly from one another. Thus, when their carrier arrives at another orchid the pollen-masses are in exactly the right position to strike against its stigmatic lobes, and so to effect cross-pollination.

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