杏吧原创

Plant life

My garden contains a number of day lilies which, as their name suggests, flower for only a very short time. Other flowers keep going for weeks. How are different plants able to adopt such different strategies? Surely any flower should have evolved to keep going until pollination has taken place and then wither?

鈥� By day lilies I assume that the questioner refers to the 20 or so species of Hemerocallis, which often die within a day of opening. The plants do, however, produce a succession of flowers, often for a period of a month or more.

One of the major evolutionary decisions facing plants is how long to keep their individual flowers open. Flowers of some species will remain open for weeks or even months. The flowers of the New Guinean orchid Dendrobium cuthbertsonii can last up to 10 months, while others, such as morning glories, remain open for only a few hours.

There are a number of advantages for plants that flower for only a short period of time. Maintaining flowers is very expensive physiologically, so the less time the flower is open, the less water and sugars it uses up. And the less time an individual flower is open the smaller the chance that predators or weather will damage it.

The principal disadvantage is that the flower鈥檚 life may be over before any pollination has taken place. If pollinators are abundant and active, the flower can be pollinated (and pollinate other flowers) almost as soon as it opens, especially if the flower is designed with quick pollination in mind. This may be the day lily鈥檚 strategy. Because its flowers are only open for one day, they can afford to direct resources into very colourful and strong smelling displays that attract pollinators quickly.

Robin Calderon, owner of the official American Hemerocallis Society display garden, describes day lilies as: 鈥淭he hussies of the plant world. They put it all out for the world to see. None of this shy elusive, only-a-bug-with-a-mile-long-snout-can-pollinate-it junk, day lilies are super sexy, they know it, they are proud of it, they flaunt it and, as a result, they finish their business in one day.鈥�

Calderon may well be right, but to truly evaluate the evolutionary trade-offs that have led to the day lily鈥檚 short flowering time, it would be necessary to study them in their native environments in China, Korea, Siberia and islands to the east of Asia, because the varieties we know so well in Western gardens have been cultivated for a long time.

Jo Lucy

Middleton, Cumbria

Wave front

My son is dismayed to have inherited my 鈥渉airstyle鈥�. We both have mainly straight hair, but there is a single, pronounced wave towards the front. As new hair grows, it starts off completely straight, and then at the 2-centimetre mark, suddenly kinks. How does hair know how to do this, and how can the trait be inherited?

鈥� Ultimately, whether your hair is wavy or not depends primarily on the shape of the hair follicles on your head. But to understand how these determine waviness we first need to understand how hair is produced.

Hair grows from tiny cup-shaped pits called follicles, buried deep in the fat of the scalp. At the base of each follicle is a structure called the hair bulb that contains the actively growing cells that eventually produce the long, fine cylinder of a hair. In the lower part of the bulb, cells are continually dividing and pushing up the cells that were formed earlier.

Once the cells have reached the upper part of the bulb, they begin to specialise and arrange themselves into six layers. The outer three layers form the lining of the hair follicle (the inner root sheath) and the inner three eventually form the hair itself. Of these three layers, the middle 鈥渃ortex鈥� provides the main structure of the hair, plus its colour and texture. It consists of helices of the hard, fibrous protein keratin. The middle cortex surrounds a central medulla and is covered by the outer cuticle, a thin, colourless layer of dead cells that help to protect the cortex.

As cells reach the mid-follicle region, they die and are compressed into what we see as a hair. The cells below continue dividing and gradually push the hair upwards and out of the skin.

The shape of each hair, viewed in cross section, depends on the shape of the hair follicle and its opening. This varies from person to person and between different racial groups. As the keratin hardens, it gets compressed into the shape of the hair follicle. The hair is then held in shape by the pattern of chemical bonds within it.

The hair follicle can be thought of as a soft rubber tube. The cells that form the hair fibre push up through the middle of the tube. If the tube is straight, its interior is circular in cross section and the hair fibre produced is similarly circular in cross section, and longitudinally straight. Now imagine the tube is bent or twisted. Close to the twist or the bend, the tube will flatten and the cross section become more oval. The greater the bend or the twist, the more oval the aperture of the hair follicle and the curlier the hair will be.

On the outer curve of the hair follicle, the cells that will eventually be keratinised to form the hair fibre are stretched. Keratinisation then fixes the cells in place so that when the hair emerges from the follicle it will form natural curls or waves. In essence, the hair follicle acts as a mould for hair shape. In people from South-East Asia, hair follicles are usually completely straight, and curly hair is rare. Among African people the follicle is often helical, producing very curly hair. Europeans have a range of different follicle forms.

The precise factors that control hair-follicle shape are not yet understood. But it is clear that genetic factors have a strong influence, so that distinctive curls are often inherited 鈥� Ed

With thanks to Kevin McElwee, Department of Dermatology, University of Marburg, Germany

This week鈥檚 questions

Tread mills

Why do car and motorcycle tyre manufacturers keep coming up with such varied tread patterns? Every time I look at a tyre, the design seems different. Why is there no standard, proven pattern?

G. Curling

Bristol

Vicious fruit

Why did pineapples evolve a fearsome array of spiny leaves that makes the large, sweet and juicy fruit almost impregnable? Surely the usual purpose of a sweet fruit is to encourage seed-dispersing animals to eat it. So what disperses the pineapple in its native range?

Colin Wilson

Darwin, Northern Territory