It’s common for people to confuse tree sap and tree resin. The two substances are significantly different, however, in several ways. All trees produce sap to a considerable degree, but resin tends to be the domain of coniferous trees.
Properties and Uses
Sap is generally a relatively clear and thin watery substance, while resin, or pitch, is often more colored. The latter tends to be viscous or extremely tacky. For example, sap that maple syrup producers use to make their product is essentially water with a very mildly sweet taste. In fact, maple sap is perfectly potable as drinking water straight out of the tap. Resin, on the other hand, is a gummy material that looks and feels more like glue. Drinking it is generally out of the question. One traditional use of resin is in the manufacture of turpentine.
Tree sap is predominantly water with various substances dissolved in it, and there are two basic forms. Sap that a tree is transporting from the soil up through its trunk and out through its leaf pores, or stomata as botanists call them, contains mineral nutrients. These are nutrients the tree has drawn in from soil water through its roots. Sap that flows from the leaves downward — generally toward the roots and other parts of the tree on its way — contains the all-important sugar that the tree has manufactured in its leaves.
Resin is very different from sap in its composition. Rather than harboring nutrients to be transported through the tree, resin is composed of compounds that are secreted or deposited in the tree. Scientists don’t all agree on the question of whether resin in a tree is a waste product or a means of protection against infection or insect attack.
A critically important tissue inside trees is vascular tissue. There are two types of vascular tissue and both contain sap. One type is the so-called xylem. a structural as well as a sap-conducting tissue; wood, essentially is xylem. Ironically, many of the critically functioning cells in the xylem are dead. The shell formed by their cell walls and their empty interiors work a bit like tiny, interconnected straws to provide structural support and to simultaneously conduct sap up through the tree. In fact, botanists have termed certain sections of the wood inside a tree “sapwood.”
As the tree ages and increases in diameter, wood at the center of the trunk — wood that used to be sapwood — becomes “heartwood.” Heartwood gradually becomes sort of clogged and ceases conducting sap while, at the same time, it accumulates some resins. So sap runs through the xylem but primarily in the sapwood, while resins tend to accumulate in the heartwood when it no longer conducts sap.
The other vascular tissue in trees is “phloem.” In a tree trunk’s cross section, the phloem is in a ring of tissue outside the xylem. It is technically part of the tree’s inner bark. Think of the phloem as food-conducting tissue. The xylem carries watery sap containing mineral nutrients upward, and the phloem carries sap, generally downward, that is laden with the all-important sugars the tree manufactures through photosynthesis.
Tree sap functions to transport vital mineral nutrients and sugars to all living parts of the tree. Because it is largely water, sap also serves to maintain what botanists call turgor pressure. During the growing season, water continually flows from the tree’s roots, up through the xylem, and to the leaves. This constant supply of watery sap inside the tree keeps leaves turgid — the opposite of wilted. Tree resin, on the other hand — since it is not conducted continuously through the tree’s vascular tissue — contributes virtually nothing to turgor pressure and wilt prevention. Resin secreted and oozing through resin ducts — and often escaping through the bark of coniferous trees — serves more of a protective function in reaction to injury or attack by insects or pathogens.