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Field Guide


Michael Wall, Ph.D.
Director of the Biodiversity Research Center of the Californias

Photo by Jon Rebman.

The Golden Barrel Cactus (Echinocactus grusonii) has dense patches of hair and spines over the growing
tip of the stem that help to reduce water loss to the environment.
Photo by Jon P. Rebman.

Water Loss:
A Necessary Evil for Plants

Jon P. Rebman, Ph.D.

In arid southern California and Baja California, water is a precious commodity and a limiting factor to most organisms, yet plants allow more than 90% of their water intake to be lost to the environment! Why? The answer lies in a fundamental difference between animals and plants.

Most animals have a heart or comparable muscular organ to pump substances around their bodies so that all of the living cells within can get the essentials necessary to stay alive and perform their functions, and so that waste products can be carried away. But plants lack a muscular organ for transporting materials. They have to be more creative in manipulating physical principles in order to get needed resources from one place to another. In plants, these functions are performed by the movement and pressures of water.

Water relations inside of a plant are rather complicated. Water pressures and gradients are necessary to move resources from places where water, nutrients, or sugars are abundant to areas where they are most needed. For example, water and minerals are absorbed from the soil through the roots of a 300-foot tree, but these materials need to be transported to the leaves in the upper canopy so that the plant can undergo photosynthesis to produce sugars (“food” for the plant) and release water and oxygen as a byproduct to the environment. Hey, take a breath and be thankful that plants release these to the environment so that they can be used by other organisms such as us!

Water is necessary for all living plant cells to function properly. It makes up the majority of cell “sap” found in each cell. The presence of water and the movement of solutes in and out of cells create water pressures at the cellular level. This function of water is essential for growth in length, and in the production of new branches, leaves, and flowers.

Water can also be cooling. When we sweat, water on the surface of our skin evaporates and cools down our temperature. Plants do this as well, but we call it transpiration, and cooling is only part of its importance. In plants, transpiration and its resulting pressures are the main processes by which water, nutrients, sugars, and wastes move throughout the plant. It is easy to see the effects of transpiration by watching how quickly a bunch of cut flowers can wilt.

Carbon dioxide is essential for plants because it serves as the carbon source for making sugars during photosynthesis. However, for carbon dioxide to get into a plant it must go into a water solution in order to pass through the plant’s cell membranes. The fundamental problem for a plant then, is that wherever water is exposed to air to get carbon dioxide it can also be lost via evaporation. Thus, the delicate equation facing almost all land plants is to limit water loss from evaporation while bringing in ample carbon dioxide to function in food-making via photosynthesis. Regulating this balance is essential to each plant’s survival.

Photo by Jon Rebman

Turtleback/Velvet Rosettes (Psathyrotes ramosissima) is a common desert species in our region that is covered with gray hairs that provide shading, reflection of ultraviolet light, and the creation of a boundary layer over the surface of the stem and leaves to reduce water loss.
Photo by Jon P. Rebman.

How do plants regulate water loss?

If plants are constantly losing water then how do our local plants deal with our dry, sometimes drought-ridden habitats? Many of our local species have evolved various physical and physiological adaptations to survive and even thrive in the dry conditions of our region. A basic example of a physical adaptation that occurs in nearly all land plants is the presence of a cuticle (a layer of wax on the outer surface of the plant) which is impervious to water. In some local plants such as the Beavertail Cactus (Opuntia basilaris) or Desert Agave (Agave deserti) found in the arid Anza-Borrego Desert, the cuticle is especially thick and strongly protective and can be seen as an obvious grayish layer over the exposed parts of the plants.

However, having a cuticle that stops water loss means that you still need a way to lose water and to bring in carbon dioxide and this is done by having stomata. Stomata are openings in the epidermis which are controlled by the swelling and shrinking of two guard cells. In this manner, the plant can open and close their stomata in response to various physical factors such as temperature and internal moisture levels.

Jon P. Rebman, Ph.D., is Curator of Botany at the San Diego Natural History Museum. Watch for Part Two of this article in next month’s Field Notes, where Dr. Rebman discusses common physical adaptations to the world’s dry areas.

Photo by Michael Wall

The Baja California Elephant Tree (Pachycormus discolor) has swollen trunks and branches for storing water reserves. Photo by Jon P. Rebman.