Tree sap is the fluid that circulates within a plant’s vascular system, serving as the internal transport mechanism for life-sustaining materials. This watery solution moves nutrients, hormones, and minerals throughout the tree, similar to blood in an animal’s circulatory system. The fluid allows the tree to draw water and dissolved inorganic substances from the soil up to the leaves. Conversely, it distributes energy-rich sugars from the leaves to every other part of the organism, ensuring growth and survival.
The Dual Transport System: Xylem and Phloem Sap
A tree utilizes two distinct types of sap, each with a specialized role, which flow through the vascular tissues: the xylem and the phloem. Xylem sap is a dilute, slightly acidic solution primarily composed of water and dissolved inorganic minerals absorbed from the soil by the roots. This sap is transported through the xylem, which consists of dead, hollow, tubular cells that form a continuous pipeline from the roots up to the leaves. Xylem sap supplies the photosynthetic machinery in the leaves with the raw materials—water and minerals—needed for producing food.
Phloem sap, in contrast, is a thicker, slightly alkaline solution rich in organic compounds like sucrose, amino acids, and hormones. This fluid moves through the phloem, a layer of living cells located just beneath the bark, and its main purpose is to distribute the energy produced during photosynthesis. Sugars created in the leaves (the “source”) are transported to non-photosynthetic parts of the tree, such as the roots, developing fruits, and new growth buds (the “sinks”). Phloem sap is a complex cocktail containing trace elements, fructans, and messenger RNA molecules that regulate growth and development across the plant.
The movement of xylem sap is predominantly unidirectional, flowing upward from the roots to the rest of the plant. Conversely, phloem sap movement is bidirectional; it can flow both up and down the trunk and branches, depending on where the energy is needed. This dual system ensures that water and minerals are continuously supplied to the canopy, while manufactured sugars are efficiently distributed for energy and storage.
The Physics of Flow: How Sap Moves Through Trees
The mechanisms driving the flow of these two saps are fundamentally different, relying on both passive and active physical forces. Xylem sap movement is mostly passive and is explained by the Cohesion-Tension Theory. This process begins with transpiration, the evaporation of water vapor from the leaves through small pores called stomata.
As water evaporates from the leaf surface, it creates a negative pressure, or tension, that pulls the water column upward. Water molecules exhibit strong cohesion, meaning they stick tightly to each other due to hydrogen bonding, creating an unbroken chain from the leaf down to the roots. This cohesive chain is pulled up by the tension generated at the top, allowing the tree to draw water from the soil against the force of gravity.
Phloem sap movement, however, is an active process described by the Pressure Flow Hypothesis, which relies on a hydrostatic pressure gradient. Sugars are actively loaded into the phloem sieve tube cells at the source, such as a leaf, which increases the solute concentration. This high concentration draws water into the phloem from the adjacent xylem by osmosis, generating a high turgor pressure.
This buildup of pressure at the source forces the fluid to flow in bulk toward the areas of lower pressure, or sinks. At the sink tissues, sugars are actively unloaded from the phloem, which causes water to move back out, decreasing the pressure and maintaining the flow gradient. This continuous cycle of pressure difference drives the translocation of sugars throughout the entire tree.
Sap is Not Resin: Understanding Sticky Tree Exudates
A common point of confusion is mistaking the internal transport fluid, sap, for the sticky substances a tree exudes when wounded, such as resin or gum. Resin is chemically distinct from sap, as it is a viscous liquid or solid composed largely of organic compounds called terpenes, which are insoluble in water. Conifers, like pines and firs, produce oleoresins, which are mixtures of volatile oils and resin acids.
The purpose of resin is defense, not transport. When a bark beetle or fungus attacks a tree, the resin flows out to seal the wound, trapping and deterring the invading organisms. This sticky, aromatic material is stored in specialized ducts within the wood and bark, becoming visible only when the tree’s outer barrier is breached.
Gums are another type of exudate, consisting mainly of water-soluble polysaccharides, and are also used by trees to seal wounds. Unlike sap, both resin and gum are defensive compounds that act as nature’s antiseptic bandage. While sap flows constantly internally, resin and gum are typically only produced when the tree is under stress or damage.