Flowers

Pollination and fertilisation are not the same event and the gap between them can be long. Pollination is pollen arriving on a stigma. Fertilisation happens later, after a pollen tube has grown down the style to reach an ovule — hours in some species, months in others.

Bee vision is shifted towards the ultraviolet: bees see UV, blue and green, and are effectively blind to red. Many flowers that look plain to us carry bold ultraviolet patterns — "nectar guides" — that point straight at the reward. Photograph the same flower in UV and it turns into a landing target.

Colour also tracks the audience. Red flowers are common where birds pollinate and rare where bees dominate, because birds see red well and most bees do not. Night-opening flowers tend to be white and heavily scented, since colour is useless in the dark and moths navigate by smell.

There are channels people cannot perceive at all. Flowers carry weak electric fields that shift after a bumblebee has visited, and bees can detect the difference and learn from it — an honest signal that the nectar has just been taken.

Flowering at the wrong time is fatal to a plant’s reproduction: too early and there are no pollinators or the frost takes the blossom, too late and the seed cannot ripen. So the decision is made from information the plant can trust, and the most trustworthy signal available is day length. Temperature varies wildly from year to year; the length of a night on a given date does not vary at all.

Plants measure it with phytochrome, a pigment that switches between two forms in red and far-red light and reverts slowly in darkness — a chemical clock reset every dawn. What it actually tracks is the length of the uninterrupted night rather than the day, which is why a single flash of light in the middle of the night can stop a short-day plant flowering altogether, and why commercial glasshouses use exactly that trick to control chrysanthemums.

• Long-day plants flower as nights shorten — spinach, barley, many temperate species flowering in early summer.
• Short-day plants flower as nights lengthen — chrysanthemums, poinsettias, rice.
• Day-neutral plants ignore the question and flower on age or size instead — tomatoes, many tropical species.
• Vernalisation is a separate requirement: some plants will not flower at all until they have experienced a winter, which is why winter wheat is sown in autumn.

Opening and closing each day is a different mechanism entirely, and it is not the plant sleeping. Many flowers close at night by growing unevenly — cells on one side of the petal expand faster than the other — or by changes in cell turgor, both driven by temperature and light. The advantage is not settled and the plausible candidates are practical: protecting pollen from dew and rain, avoiding wasting nectar on animals that are not useful pollinators, and keeping the reproductive parts warmer.

A flower carrying both male and female parts has an obvious problem: it is very easy to fertilise itself, and inbreeding costs. Plants solve it in several ways at once — maturing anthers and stigma at different times, separating them in space, putting male and female flowers on separate parts of the plant or on separate plants entirely, and recognising and rejecting their own pollen chemically at the stigma.

Self-pollination is not always a failure, though. Where pollinators are unreliable, a plant that can set seed alone is better off than one that cannot, and many species keep the option in reserve — some produce closed flowers that never open and self-pollinate inside.

• How do multiple signal channels combine in a pollinator’s decision?

  Why it matters: Colour, scent, shape, texture and electric field are all detectable; whether they reinforce, substitute or are ranked is largely unknown.

• Why did flowering plants diversify so explosively and so fast?

  Why it matters: Darwin called it an abominable mystery, and while pollinator coevolution is part of the answer it does not fully account for the rate.