Beginnings and the Seed part 2

Almost every plant produces many thousands or even millions of seeds during its lifetime, but to ensure the survival of the species it is necessary for just one of these to reach maturity. This vast wastage is in reality a form of insurance as at least some should survive to pass the seedling stage. Generally the smaller (and therefore the more vulnerable) the seeds, the greater is the number produced.

As well as a store of food and a protective coating, seeds frequently show adaptations which help them to become more widely distributed. Many seeds are dispersed by the wind. The tiniest seeds of all are those of the orchids, many of which are no larger than a pollen grain and float in the air. Heath Spotted Orchids (Dactylorrhiza maculata) have an average of 6,200 seeds in every pod. As anything up to thirty pods may ripen on a plant, that one orchid has a seed production of about 186,000 seeds. Tropical orchids are even more profligate and a single pod of Maxillaria can contain 176,000 seeds.

Wind dispersed seeds which are larger than this usually have some devicewhich catches the wind; sometimes this is feathery, in other plants it is solid, acting as a wing or propeller. Downy or feathery fruits float in the air and are sometimes carried for long distances. Those of Rosebay or Fireweed (Epilobium angustifolium) are also very_ small and light but others have a more distinctive parachute shape with the seed attached on a stalk. This gives the seeds a definite centre of gravity and they are capable of ‘hanging’ in the air. The familiar dandelion ‘clock’ is made up of a number of these parachutes which drift away as they are blown.

The feathery part of these seeds arises in a number of different ways. In Red Valerian (Centranthus ruber) the calyx changes to down as the fruit natures while in Pasque Flower (Pulsatilla vulgaris) and Old-man’seard (Clematis vitalba) it is the style which elongates and develops i covering of hairs. In the Rosebay and poplars (Populus), the hairs ;row upon the seed itself. Many tree seeds have membraneous wings which help the seeds to fall away from the parent tree. Birch (Betula) seeds are relatively light, weighing from 0.17 to 0.5 mg and they have a small wing on each side. Pine (Pinus) fruits are heavier, up to 7 mg but their single wing enables them to be carried up to 30 metres from the parent tree. With wings of this sort, even large fruits are able to make use of air currents and wind fordispersal. Those of the Hornbeam (Carpinus betulus) weigh about 40 mg, while fruits of European Ash (Fraxinus excelsior) and Field Maple (Acer campestre) weigh 70 and 75 mg respectively. To keep these fruits airborne their centre of gravity is to one side and_ the slightly twisted wing acts as ^-a Propeller sending the seed spinning to the ground. In the case of the limes (Tilia) all the fruits from each inflorescence are joined by short stalks to a bract which itself acts as a wing.

Another group of plants, particularly those which grow close to the ground, have seeds which are dispersed mechanically by animals. Many develop hooks which catch in the fur or feathers of passing creatures, eventually falling off far from the parent plant. These hooks can be small as in Cleavers or Goosegrass (Galium aparine), giving the seeds a roughened feeling, or larger and stronger as in burdocks (Arctium) and the burs (Acaena). The seeds of Water Avens (Geum rivale) have a single hook formed from the style.

Many fruits are eaten by birds and sometimes by other animals. In these cases it is the colourful outer layers which attract the creature’s attention. Colours in both fruits and flowers are formed either by the presence in the cells of plastids which are visible through a microscope as separate entities, or by a pigment which is present throughout the vacuole of the cell. The most common plastids are the chromoplasts which contain the pigments carotene and xanthophyll. These are red and yellow, and in combination they can give all shades of the two colours and orange. The red colour of the rose hips and the orange coloured fruit of the Sea Buckthorn (Hippophaë rhamnoides) are derived from carotin and xanthophyll as are the petal colours of Nasturtium (Tropaeolum majus).

The commonest of the pigments found in solution are the anthocyanins which are chiefly responsible for blue, purple, brown, black, and those shades of red which lean towards magenta and purple, as distinct from the scarlets and orange-reds of carotin. The exact shade changes as a result of the acidity or alkalinity of the cell sap; where it is acid the anthocyanins give a red colouring, where alkaline, blue. The bright red of the bedding geraniums (Pelargonium), the blue of the Cornflower (Centaurea cyanus) and the violet of Pansies (Viola) are formed by this group of pigments while they also give the red of a beetroot (Beta vulgaris) and the near black fruits of privets (Ligustrum) and some grapes (Vitis). In some flowers, for example the forget-me-nots (Myosotis), the flowers open pink, but change to blue as they develop.

Although there are many white flowers and fruits there is no responding pigment. In fact the white is caused by minute air aces between the cells which reflect light. This reflected light is en by man as white. Many plants have white-flowered forms and these are simply the result of the absence of pigment. Amounts of pigment do vary greatly from one plant to another, the Yarrow chillea millefolium) being a good example. Most of the flowers are white but there is normally some anthocyanin present and if this mount increases they have a pink coloration which in extreme examples can be quite dark.

Recent research has found that birds can see red most easily and the majority of berries and fleshy fruits are of this colour, for example in Woody Nightshade (Solanum dulcamara), Lily-of-the-Valley (onvallaria majalis), Barberry (Berberis vulgaris), Redcurrant Ribes rubrum), Dog Rose (Rosa canina) and Mezereon (Daphne zereum). The fruits of many plants which appear black, such as wets, Blackthorn, Spurge Laurel (Daphne laureola) and Cherry aurel (Prunus laurocerasus) are in fact a very dark purple.

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