Hydrophytes and xerophytes

Last time

Before we start: last time, in Variation and mutation, you learned these points. Here they are again on one card. Keep them in mind, because today builds on them.

Two extremes of water

A water lily floats on a pond with water on every side. A cactus lives in a desert where it might not rain for a year. Both are plants, but they have opposite problems. Today: how hydrophytes cope with too much water, and how xerophytes survive with too little.

Two words

A hydrophyte is a plant adapted to live in water, or in very wet places. Hydro means water. A xerophyte is a plant adapted to live in dry places. Xero means dry. Each has adaptive features: inherited features that help it survive in its habitat.

Why water loss matters

Plants lose water vapour from their leaves, mainly through the stomata. This is transpiration. In a desert there is little water to replace it, so a xerophyte must cut water loss to a minimum. Heat, wind and a large leaf surface all increase water loss.

Xerophyte features: leaves

A thick, waxy cuticle on the leaf reduces evaporation from the surface. Small leaves, or leaves reduced to spines, give a small surface area for water to escape. Leaves may roll up, and hairs trap a layer of moist air, so there is less water loss.

Xerophyte features: stomata and roots

Few stomata, often sunken in pits, reduce the water vapour that can escape. A deep root, or a wide shallow root system, collects water from a large volume of soil. Succulents like the cactus store water in thick stems, and may open their stomata only at night.

Hydrophytes

Hydrophytes have plenty of water, so they need little protection from water loss. They have a thin or no cuticle. Many have large air spaces in the leaf and stem, which help them float and carry oxygen to the roots. Floating leaves have many stomata on the upper surface, in contact with the air.

Hydrophytes: roots and stems

The roots are small, or absent, because the plant takes water in over its whole surface. The stems are flexible, so they bend with moving water, and are supported by the water. There is little xylem and little lignin, since water supports the plant and does not need long transport.

Surface area and volume

A big flat leaf has a large surface area for its volume, so it loses a lot of water. A cactus spine has a tiny surface area. If the leaf surface area is cut from a hundred square centimetres to ten, the area for evaporation is ten times smaller. The cactus does its photosynthesis in the green stem instead, which has a smaller surface area.

The trade-off

Fewer stomata save water, but they also let in less carbon dioxide, so photosynthesis is slower. That is why many desert plants grow slowly. Survival matters more than speed in a dry habitat. Opening the stomata only at night balances saving water against getting carbon dioxide.