Albert French, age 12, of Victoria, B.C., for his question:
What use is tree sap?
Sap is the life blood of the tree. Like your blood, it carries dissolved food to the tissues where it is needed. The circulation system of a tree has no heart to pump its life‑giving sap. Nevertheless, it is a complex, well‑run system of circulation.
The sap is the‑ watery,juice which oozes out when we cut a living twig or leaf. Your body has two kinds of blood in its vessels. There is the used blood returning to the heart and. lungs and the fresh red blood pulsing from the heart and lungs charged with oxygen to feed the tissues. There are also two kinds of sap flowing through the tree. Each has its own circulatory system.
One kind of sap is water soaked up by the roots from the ground. It is called crude sap and it contains salts and other chemicals which the water has dissolved from the soil. The other is called elaborated sap. It bears dissolved carbohydrates, fats and proteins down and crosswise to the tissues, which need these foods.
Apart from having a heart, the tree's circulatory system is as complex as yours. It too is a system of veins. It is a connecting system of tube cells called vascular bundles. In your body the blood vessels carrying newly charged red blood are the arteries. In a tree, the vascular bundles which carry crude sap are called xylem cells‑‑pronounced zy‑lem. Those carrying elaborated sap are the phloem cells, pronounced flow‑em.
The heart of a tree trunk is dead wood and plays no part in the circulation of sap. The vascular system is at the outer edge of the trunk. The phloem cells are at the outer edge. The xylem cells are further inside, The two systems are separated by a miraculous layer of cells called the cambium. The cambium cells grow by dividing in two. The new inside cells are xylem cells, the new outside cells are phloem cells.
As you would expect, the xylem system conducts water from the soil clear up to the top of the tree. This is aided by nature's law of osmosis, At the end of the twigs and branches, the crude sap enters the leaves. The green leaves are factories where plant sugar is made from crude sap, air and sunlight. This is done by the miraculous process of photosynthesis, which will be explained in a later article.
The simple plant sugar is added to the liquid sap. Other plant processes turn this sugar into all sorts of other products. They too are dissolved, and the sap becomes richer and richer, This elaborated sap is carried down and sideways by the phloem vascular system. As it flows by, each cell takes what it needs of the dissolved chemicals.