Sap

Sap is the fluid that circulates within vascular plants, supporting growth, metabolism, and storage. In common usage, sap refers to two distinct fluids found in different transport systems: xylem sap and phloem sap. Xylem sap is primarily water with dissolved minerals absorbed from the soil. It is transported from roots to shoots through xylem vessels and tracheids, mainly driven by transpiration and aided by root pressure and cohesion-tension forces. Phloem sap is a nutrient-rich liquid carried in the phloem from photosynthetic tissues to non-photosynthetic or storage tissues. It contains sugars (primarily sucrose), amino acids, organic acids, hormones, and minerals, along with water.

The transport mechanisms of the two saps differ. Xylem movement is largely a bulk flow process driven by evaporation at the leaves, which creates a negative pressure that pulls water upward. Phloem transport follows the pressure-flow hypothesis: sugars are loaded at source tissues, increasing osmotic pressure and drawing in water to create positive turgor that pushes sap through sieve tubes toward sink tissues where sugars are unloaded and used or stored. The composition and volume of sap vary with species, tissue type, developmental stage, and environmental conditions such as light, temperature, and water availability.

Sap has practical significance in nature and human use. Maple sap, collected from sugar maples in spring, is processed into maple syrup. In agriculture and forestry, sap composition can affect plant health, fruit quality, and growth. Sap can also play a role in the spread of pathogens or contaminants, as it can transport substances within the plant.