Will you have beans with your nurdles?

Cloud permitting, the Harvest Moon on Wednesday night will be hoisted up over the mountain like a Chinese lantern, a deeply-appreciated signal that the summer is definitely over. The next big tides aren't due until the equinox, later in the month, but then they will scrub out the last of the holiday footprints and leave the strand shiny and seaweed-smelling for the proper celebration of autumn.

After the rather empty tide-lines of the summer calms, there might even be a gale to bring us nurdles and thus, perhaps in their company, some more unusual drift-seeds of the season.

Nurdles (it is a word - try it on the Internet) is now, apparently, the accepted name for the tiny, translucent pellets in which raw polypropylene is shipped around the world. And wherever these feather-light, drum-shaped plastic fragments fetch up in a sandy corner of a shore, you are likely to find the seeds of sea pea, sea bindweed or morning glory, which are much the same size and weight.

As late as a couple of decades ago, driftseeds reaching the beaches of Ireland were still thought to be very rare and their finders thereby blessed with luck. Now, as botanist Charles Nelson relates in his new book, Sea Beans and Nickar Nuts, the sharp eyes of more ecologically dedicated beachcombers in Ireland and Britain sometimes find them by the hundred and in many different species.

The 20-odd years of his research into drift-seeds and their history coincided with his tenure as taxonomist at the National Botanic Gardens. It is also the span of my own regular beachcombing, which has produced a dishful of sea-beans, along with a row of ocean-going coconuts in varying stages of undress. The biggest of the beans is Entada gigas, the "sea-heart", up to six centimetres across and with a tough, dark skin that, with handling (and all these beans invite fondling, like worry-beads) takes a polish like Moroccan leather. It falls from the metre-long pod of a rampant liana in tropical America and arrives here by way of the Gulf Stream and North Atlantic Drift. Horse-eyed bean, Mary's bean, sea purse and nickar nut bob along the same way, taking perhaps 400 days to make the transatlantic journey.

A discussion of drifts and currents adds to the fascination of Sea Beans and Nickar Nuts, a book that any serious beachcomber will prize. It is 300 years since the Irish-born physician and botanist, Hans Sloane, deduced the link between the tropical plants of the West Indies (which he had seen) and the "beans" turning up on the coasts of Ireland and Scotland. Charles Darwin was another scientist hugely intrigued by the significance of drifting seeds for the spread of plants around the world.

Dr Nelson describes nearly 40 species, including such rare Amazonian fruits as the golf-ball bean and the blister pod. I am also proud to see the only specimen of the box fruit, Barringtonia asiatica, so far recorded from a European shore. Proud, because I picked it up myself in 1985 on a lonely winter beach in the Inishkea Islands off north Mayo: a four-sided fistful of husk with a seed rattling somewhere at its heart. Wendy Walsh's drawing is typical of her exquisite work for the book.

Barringtonia is a tropical tree with spectacular white flowers that open only for one night, pushing out a brush of pink stamens for pollination and dropping them by dawn. The fruits that swell from this brief idyll can float on ocean currents for more than 15 years and, in the Pacific, were among the first colonists of Krakatau, the new island born of volcanic eruption.

But to colonise, of course, drift-seeds must germinate and prosper: what chance is there of growing plants from those that reach Ireland? The only sure way to test if a seed will grow is, as Nelson says, to try it and see - thus, quite probably, losing your lucky bean in the process.

Entada, for example, often arrives in a viable state (but not if it rattles) and puncturing the coat, just enough, with a hacksaw, soaking it in fresh water for a day and planting it in sandy compost kept at about 27 degrees may well wake the jungle vine sleeping within.

But then one of two things happens. Either a fungus leaps upon the seedling (our own experience) or, if one has watered in the bean with fungicide, a pantomime beanstalk begins to rocket to the ceiling. ("It is probably best," says Charles Nelson sagely, "not to attempt to germinate seeds unless you have a high-roofed conservatory in which to accommodate a gigantic vine.")

A window sill, then, is not the best place for such experiments, being cold and draughty as well as too small. A constant 25 degrees and high humidity is needed for even the climbers that are content with reaching up the curtains and trailing around the walls. In growing experiments conducted at Glasnevin some years ago, seedlings of sea purse and horse-eye beans both topped 1.5 metres within 17 weeks. The nickar nut (like an ash-coloured acorn), on the other hand, can be tamed into a prickly bush with spikes of pea-like flowers. And the temperate species (tide-line tinies such as the sea pea that have probably made it from North America), will grow and flower outdoors in well-drained patio pots after a judicious rub inside with a fold of coarse sandpaper.

On the east coast of the United States, as Nelson reports, the sale of drift-seeds is now a regular business, and even on this side of the ocean specially-imported sea-hearts are on sale as "lucky bingo sea-beans" in themepark and seaside shops. For the true beachcomber, lone hunter of the tide-line, this book is a timely counter to such crassness.

Sea Beans and Nickar Nuts is available from the Botanical Society of the British Isles, c/o Summerfield Books, Main Street, Brough, Cumbria CA17 4AX, UK, at £13.95 in the UK.