Reading: Matching Features: Question 4

Mangrove forests growing along tropical coastlines are rarely made up of a single tree species; instead, several distinct species typically grow in bands running from the water's edge inland, each having evolved a different way of coping with salt water that would kill most ordinary trees.

Closest to open water, standing directly in the tidal zone, is usually the red mangrove. Its roots contain a specialised membrane that filters out roughly ninety per cent of the salt in seawater before it ever enters the plant's tissue, a process known as ultra-filtration. Because the surrounding mud is almost permanently waterlogged and low in oxygen, red mangrove also grows arching, above-ground "prop roots" that lift much of the root system clear of the anaerobic mud and allow it to take in air directly.

Slightly further inland, in a zone still flooded by most high tides but for shorter periods, black mangrove dominates. Rather than filtering salt out at the root, it allows salt to enter and then actively pumps the excess back out through glands on the surface of its leaves; on a hot day, the excreted salt often dries into visible white crystals that can be brushed off the leaf. Black mangrove also sends up thousands of narrow, pencil-like roots called pneumatophores that poke directly out of the mud to draw in oxygen.

Further inland still, in ground that floods only during unusually high spring tides, grows white mangrove. Like black mangrove, it excretes excess salt through glands, though its glands sit at the base of each leaf stalk rather than across the whole leaf surface, and it tends to accumulate noticeably less salt in its tissues overall than either of the two species growing closer to the water.

At the landward edge of the forest, in ground the tide reaches only during exceptional storms, buttonwood grows. Unlike its three neighbours, buttonwood has no dedicated salt-excreting glands and cannot filter salt at the root; it survives in this outer zone only because rainfall and freshwater runoff there keep the soil's salinity relatively low compared with the tidal mud closer to the water.

Mangrove ferns, an epiphytic species that grows attached to the trunks of several of these trees rather than rooted in the mud itself, are common throughout the forest but were not part of this particular salinity study.

List of Species A. Red mangrove B. Black mangrove C. White mangrove D. Buttonwood E. Mangrove fern

The following statements describe traits found in a mangrove forest. Match each statement with the correct option, A–E. NB You may use any option more than once.

1. Filters out most of the salt in seawater at the root before it enters the plant's tissue.
2. Grows arching roots above the mud surface to obtain oxygen in permanently waterlogged, low-oxygen ground.
3. Excretes excess salt through glands spread across the surface of its leaves, sometimes leaving visible crystals.
4. Sends up narrow, upward-pointing roots that emerge directly from the mud to take in air.
5. Excretes salt through glands located at the base of each leaf stalk rather than across the whole leaf.
6. Accumulates less salt in its tissue than the two species that grow closer to the water.
7. Has no dedicated structure for excreting or filtering out salt, and instead relies on rainfall keeping the local soil less salty.