The two magnificent pine trees stand next to the sandy main walk in the Cambridge Botanical Gardens. One tree with its dark, short needles and its pointed, tapered shape, looks like a picture-book Christmas tree. Its neighbour, with its long, pliant needles, has an open canopy with graceful curved branches sweeping earthwards.
But the plaque on both trees says Pinus Nigra. And these aren't just any old black pines, either. These trees were planted in 1846 by the botanical garden's founder, Professor John Henslow, and they may have partly inspired his famous pupil, Charles Darwin, to write his revolutionary book on natural selection, Origin of the Species, in 1859.
The dense, triangular Pinus Nigra is from Austria and the more open Pinus Nigra is from the Mediterranean shore. The former's pointed shape and short branches allows the snow to slide off it, while the latter's big loose branches have evolved to suit a warmer climate. This amply illustrates Darwin's theory that species adapt to their environment.
The two trees offer some interesting lessons for children. The younger ones can count the differences between the two trees, while the older ones can speculate about whether they are from the same species. "You can get children to see the relationship between variation, and climate and geography, which leads to natural selection," says the director of the gardens, Professor John Parker.
In the garden's magnificent Victorian greenhouses, the collection of beautiful but deadly carnivorous plants also shows how species change to survive. "They are like fly paper," says Professor Parker. An unwary insect venturing too close is trapped in a maze of sticky hairs, or on flat sticky leaves.
There is no going back for the fly which ventures into the triffid-like, red veined trumpets of a Pitcher plant, attracted by its dark red cap, which looks like rotten meat. Trapped by the slippery sides, the fly falls into a watery grave. The plant then converts the protein in its body to nitrogen, often in short supply in their natural habitat.
Teachers can also get children thinking about the different ways that a human and a plant might defend themselves from attack. A child confronted with danger might say, '"I will run away, climb a tree or hide behind a bush'," says Professor Parker. But immobile plants have had to develop spines, thorns, poisons or sticky gum to repel attackers.
Some of our everyday sweets and foodstuffs were evolved as a plant's defence. If a predator damaged Manil Kara, the original chewing gum plant, its chewy defences would gum up animal or insect jaws.
The coffee plant's caffeine kills insects or animals which might eat its beans - the caffeine is most concentrated in the seeds, which are the key to the next generation. Once again, we return to the evolutionary survival principle.
The botanical gardens, originally located on a five-acre site in central Cambridge, were modelled on Chelsea Physic Garden in London, and the healing herbs and plants grown there were used to teach medical students.
Professor Henslow obtained the present 40-acre site for an arboretum, and the gardens have developed from there. A pound;16.5 million education centre with a plant laboratory, classrooms, auditorium, exhibition gallery and library is planned for the future.
The gardens offer curriculum-linked courses and special events tailored to all key stages. Call education officer Christine Preston for further details. Tel: 01223 3318756; email: firstname.lastname@example.org; Cambridge University Botanic Garden; Cory Lodge, Bateman Street, Cambridge CB2 1JF; www.botanic.cam.ac.uk