Plants have two main transport systems. The xylem transports water and minerals from the roots upward to the leaves. The phloem transports sugars, which are the food produced during photosynthesis, from the leaves to other parts of the plant. These two systems serve different functions in the plant.
When plants are placed in colored water, the water enters through the roots and travels upward through the xylem vessels. The dye in the colored water stains these xylem vessels, making them visible. However, the phloem, which is the food-carrying tube, remains unstained because it doesn't transport water from the roots. Instead, phloem transports sugars from the leaves to other parts of the plant. This is why we see color change in the xylem but not in the phloem.
Under a microscope, we can see the structural differences between xylem and phloem that explain why colored water stains only the xylem. Xylem vessels are dead, hollow tubes with thick, lignified cell walls arranged in rings or spirals. When colored water is absorbed, it flows through these hollow tubes and stains them. In contrast, phloem consists of living cells with cytoplasm and thinner cell walls. Phloem transports sugars produced in the leaves, not water from the roots. This is why the colored water doesn't enter or stain the phloem tissue, leaving the food-carrying tubes unchanged in color.
The direction of flow in xylem and phloem also explains why colored water stains only the xylem. In xylem, water flows in one direction - from the roots upward to the leaves. This flow is driven by transpiration, which is water evaporation from the leaves. When colored water is absorbed by the roots, it follows this upward path through the xylem. In contrast, phloem has bidirectional flow, moving from source to sink. Sources are areas that produce sugars, like mature leaves, while sinks are areas that use or store sugars, like roots, fruits, or growing tissues. This flow is independent of water uptake from the roots, which is why the colored water doesn't enter the phloem system.
To summarize why the food-carrying tubes don't change color when plants absorb colored water: Plants have two main transport systems - xylem and phloem. The xylem transports water and minerals from the roots to the leaves, while the phloem transports sugars, or food, from the leaves to other parts of the plant. When plants are placed in colored water, the water enters through the roots and travels upward through the xylem vessels, staining them with color. The phloem, however, doesn't transport water from the roots. Instead, it transports sugars produced in the leaves. Since the colored water doesn't enter the phloem system, the food-carrying tubes remain unchanged in color. This difference in function between xylem and phloem explains the observation that only the water-carrying tubes change color in the experiment.