Beginnings and the Seed part 3

Many of these seeds are chewed up and destroyed by animals and hers are broken down in the alimentary canal of birds, particularly of chickens, pigeons and seed eaters with strong beaks such as most the finches. A great number, however, have hard enough coats to remain intact while the fleshy parts are digested and are finally voided by the birds, sometimes after being carried for long distances. Birds which do not have the powerful beaks of true seed eaters do least damage; 85 per cent of those extracted from Song Thrush (Turdus philomelos) droppings and 75 per cent of those from a Blackbird (Turdus merula), for example, germinated when they were sown. In fact the seeds of many plants will germinate more readily after they have passed through a bird’s digestive system. This has been proved so in the case of Barberry, some currants, nettles (Urtica), Knotgrass (Polygonum aviculare) and pigweeds (Amaranthus).

Many of the seeds which have no obvious means of dispersal are, in fact, distribu^ted by- animals. The large Free seeds such as acorns, beech mast and chestnuts are so heavy that they fall with no possibility of moving beyond the canopy of the parent tree. Because of their food content, however, they are collected by such creatures as jays (Garrulus) and squirrels (Sciurus). Squirrels are great hoarders, carrying the seeds away and burying them. Surprisingly they are very inefficient at rediscovering their larders and much of their buried store remains in the ground to germinate the next spring.

Other seeds are dropped, particularly by jays and crows (Corvus) and again they remain in the leaf litter ready for germination. The seeds of the Mistletoe (Viscum album) are surrounded by a very sticky substance which the birds do their best to remove from their beaks by wiping it on the rough bark of a tree, thus unintentionally finding the perfect place for them to germinate. Cyclamen seeds have a sticky coating which makes them attractive to ants and they also are responsible for the dispersal of the seeds of violet, Lesser Celandine (Ranunculus ficaria) and Asarabacca (Asarum europaeum).

0 Many plants have their own mechanism for seed dispersal rather than relying totally upon outside influences. Poppies have tiny seeds which when ripe are freed from the walls of the carpels in the capsule. The lid of the capsule is slightly overhanging, and beneath it is a number of small holes. As the whole seed head is shaken by the wind so the seeds are shaken out like pepper from a pot. Other plants open their capsules explosively, literally catapulting their seeds Out., Species of geranium have five seeds each in a separate carpel to which the style remains joined. As they ripen, so the style contracts and each section breaks free at the base, catapulting out its seed as the style curls rapidly upwards. As they dry, the pods of Gorse (Ulex europaeus) twist and finally explode, flinging the seeds far from the plant. The sharp cracking noise this makes is familiar to everyone who has walked on a gorse-covered common in late summer.

The mechanism for those species with dry fruits is usually activated by water, or lack of it, but there are also plants whose pods open to expose the seeds only when it rains. Several evening primroses (Oenothera) come into this category. In other plants, such as Marsh Marigold (Caitha palustris), the seeds are washed out of the capsule, falling onto wet ground where they eventually germinate. Conversely, some dry ground species such as asters keep their heads of plumed fruit closed in damp weather as moisture would reduce the efficiency of their mechanism for distribution.

Water plants show very distinct adaptations for dispersal. The fruit of the White Water Lily (Nymphaea alba), which is in fact a berry, develops under water. When it is ripe it splits open to release the seeds. These are covered with a tissue of mucilaginous cells which contain a great number of air bubbles. Held up by these bubbles the seeds float downstream until in time the bubbles burst and the seeds sink to the bottom, ready to germinate. Salt water plants also make use of the sea for transport as do a number of land plants whose chief habitat IS near the coast. The most famous is probably the Coconut Palm (Cocos nucifera) whose fruit is capable of floating for Several weeks. The seed inside is however viable for only a short time, so the nuts which travel long distances are dead by the time they reach land. Other fruits and seeds live longer and have been known to cross oceans.

This is only a sample of the many ways in which seeds are distributed and it is always worth noting the mechanisms used by our most successful weed species to see what it is about them that has proved so advantageous. In many cases the ability to flower and fruit whenever the weather is suitable, rather than once a year, is a prime factor, but seed distribution plays its part too. Chance may play an important part, as in the case of the Oxford Ragwort (Senecio squalidus) whose spread throughout Britain was undoubtedly facilitated by the miles of disturbed land which accompanied the making of the railways. Today its cheerful yellow flowers are a feature of most railway yards. Pineapple Weed (Matricaria matricarioides) has taken similar advantage of the treads of rubber tyres. A hundred years ago this plant was a rarity and its seeds were no doubt moved around in the mud on horses’ hooves. It then found a wider disseminator in bicycle tyres, but came into its own with the internal combustion engine and tyres with deep treads. It is now abundant throughout much of the world.

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