And the winner is ...

Leafiness is what you need from houseplants, even if, like cyclamen, they produce flowers as well. Greenery makes even the dingiest cell seem more hospitable, as our eldest daughter discovered when she started her university term in a room that would have had prison reformers on the phone to the News of the World. But among her luggage was Matilda, a large prayer plant (maranta) and an even bigger rubber plant which for some reason has never acquired a name. That is unusual in our family, where even visiting spider plants get christened. The plants did more than posters, Indian throws or music to make her room feel like home.

First you've got to get them there, and in a car heaped with tottering piles of Guatemalan jerseys and jars of Nescafe, it's not easy to find room for a prayer plant in full fig. But plants are so forgiving. The ones scattered about our home could be basking in a Malay jungle, or cruising through the undergrowth of a Dominican rainforest. Instead they are stuck on the window ledge of a freezing cold house and expected to make the best of it.

Not surprisingly, some cope better than others. Begonias, the leafy kind, do well with us because they positively enjoy the cool. And the rather low light that there is, for instance, in the old, shallow stoneware sink on the landing. When we came, this was the only source of water upstairs, a sink 2ft 10in long by 1ft 7in wide and only 4in deep. It is set under an east-facing sash window and, from the beginning, has been filled with houseplants.

I started with an old Begonia maculata that my aunt threw out when she moved house. I've lost count of the number of times I have divided it up and passed it on to friends. Sometimes it spends the summer outside in the shade, beside the back door. It makes a tall plant, the leaves growing on strong, bright green canes. When it gets too leggy, I cut a few of the tallest stems down. You can root the tops to make new plants. The leaves, lopsided in the usual begonia way, are about 1ft long and spotted all over with silver, the spots arranged rather evenly, large and small alternating. It's flowering now, with drooping clusters of waxy pink flowers.

It gets fed when it gets watered, which is not more than once every couple of weeks, but I would guess that in our house, with stone floors and no central heating, it loses less water than it would in a close-carpeted, centrally heated room. It likes damp, that is, a damp atmosphere. Like most houseplants it loathes being wet at the roots, and it doesn't seem to like misting, either. Not that I've ever tried, but someone who had, told me it made the leaves die back.

Begonia manicata is much more compact, with furry, fat stems slowly winding round and round themselves like snakes to produce a low mound of handsome, staggeringly glossy leaves. Each one could star in a Mr Sheen commercial. They are dark, bronzy green, with ruffs of strange little red hairs round the margins and on the undersides. They are the kind of leaves that I'm tempted to sink my teeth into. The plant heaps itself up in an elegant way, each leaf fitting neatly into a space on the same plane as its neighbours. The stems are spotted with red, and are hairy like the leaves. The flowers are carried well above the leaves on strong, upright stalks. They appear in late winter.

Two new begonias have recently joined the trough. One I bought as `Gloire de Lorraine' but it obviously isn't, as that variety has bright green leaves and single pink flowers whereas my plant has dark, lustrous bronze leaves with the dull sheen of the best sort of satin. It's been flowering over the last couple of months (the real `G de L' is winter-flowering) bearing the flowers like cherry blossom at the shoot tips. They are bright pink and heavily double.

David Rhodes of Rhodes & Rockliffe, which specialises in leafy begonias (they put up a stunning display at Chelsea this year), thinks that mine must be `Lady France', an old variety popular in Victorian conservatories. The growths are quite lax and when the flowers have finished, I think I'll pinch back the shoots to make them break into more growing points. The more growing points, the more flowers.

The other new begonia is an "eyelash" type, so-called because the leaves are heavily fringed with white hairs. The stems are covered with the hairs, too, looking like wool when the leaves are unfurling and the stems short. As the stems grow, the hairs space out and become less dense. The leaves are crinkled, with points like an ivy leaf, rather than the smooth, rounded shape of B. manicata and `Lady France'. They are dark, with a paler splash in the centre. The one I have is `Beatrice Haddrell'. It is winter-flowering. It's a more compact plant than the others, about 8in high and the same wide.

I like the begonias because they like us, are undemanding and suit the particular conditions of the house - and because I've never seen a bug on them. The asparagus fern, Asparagus densiflorus Sprengeri, which stands behind them in the sink, collects aphids as though its life depended on it (rather than the reverse), but they never cross over on to the begonias.

If plants collapse it's likely to be because they have been overwatered. Thick, sappy stems are prone to fungal attack if the compost is too soggy. If leaves are tinged with yellow (or, in the tall, cane types, too much red), you may not be feeding the plants enough. They do best if they are repotted in fresh compost every spring.

Now, reading Eco-Friendly Houseplants, I learn that my begonias are doing me good as well. They have a high transpiration rate, which means they suck in nasties from the atmosphere at a greater rate than many other houseplants. "As water moves rapidly from the soil surrounding the roots up through the plant, air is pulled down around roots adding nitrogen gas and oxygen to the soil," writes the author, Dr BC Wolverton. "Through a biological process called nitrogen fixation, certain microbes can convert atmospheric nitrogen gas into nitrate, a chemical that plants use as a nutrient."

Because air inside buildings is naturally dry, a high transpiration rate in a plant means that more air (and toxins) go to the root zone where microbes absorb and convert them into food.

Dr Wolverton has spent most of his working life as a NASA scientist, researching closed life-support systems for future space stations. In this book, he assesses 50 houseplants according to their ability to remove chemical toxins from the air, ease of maintenance, resistance to pests and transpiration rate. Then he marks them out of 10. The begonias got 6.3 points.

The best-performing plants, according to his criteria, are the areca palm, Chrysalidocarpus lutescens, and the lady palm, Rhapis excelsa. Both scored 8.5 points. The areca palm transpires two pints of water every day and is the best known plant for removing the toxins associated with air indoors (most commonly formaldehyde, present in paper towels, floor coverings, carpet backings, plywood, chipboard etc).

`Eco-Friendly Houseplants' is published by Weidenfeld & Nicolson at pounds 14.99.

A wide range of begonias is available by mail order from Rhodes and Rockliffe, 2 Nursery Road, Nazeing, Essex EN9 2JE (01992 463693). Send two first- class stamps for a copy of their catalogue. The nursery is open by appointment only.