by Keith Hamburger

Created on: May 29, 2008   Last Updated: September 19, 2008

Plant sap serves much the same purpose as blood does for animals, carrying water and nutrients throughout the plant.

Higher order animals, vertebrates in particular, use blood to carry nutrients to cells and carry waste products from cells. In animals, nutrients come in through the stomach and intestines and oxygen from the lungs. From there the blood carries those nutrients and oxygen to all of the cells in the body. Individual cells process those nutrients to serve different purposes in keeping the animal alive. Some cells, the muscles, provide motion. Others, neuron and sensory cells, allow the nervous system to operate. Still others, the blood cells, help to protect from diseases and carry the oxygen and nutrients needed by all cells.

Plants circulation is much more simple. Nearly all of the work to keep the plant alive takes place in the leaves or other cells containing chlorophyll. The chlorophyll converts carbon dioxide and water into sugar through photosynthesis. That sugar provides nutrients to the entire plant. Water, along with other nutrients, is carried to the leaves from the roots by the plant sap. The sugar created by the leaves is carried to the rest of the plant by the sap as well.

Like the platelets in animal blood that creates clots to protect the animal from injuries, sap can serve a similar purpose along with carrying water and nutrients throughout the plant. Animal blood clots to create a scab to promote healing. Plant sap seeps out of wounds and dries creating a similar protective coating.

Sap is carried throughout the plant by capillary motion and evaporation of excess water from the leaves. As the water evaporates it draws more water behind it. The sap is drawn up along with the water through tiny tubes in the plant. This is the force that allows the water and nutrients to move from the roots to the tops of even the tallest trees.

Another purpose served by plant sap is to protect plants in cold climates from freezing. Perennial plants in cold weather can change the makeup of the sap to act as an anti-freeze. This allows them to survive year after year. In warmer climates this isn't necessary. And, annual plants don't survive freezing; they come up from seeds every year when the weather warms.

Sap is essential to the survival of plants. Without the special characteristics of sap, and the flow of sap from one part of the plant to another, all of the green plants we know couldn't exist.

Learn more about this author, Keith Hamburger.
Click here to send this author comments or questions.