Biology · Organisation. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.
A leaf is a plant organ made of several tissues, each suited to its job. Most photosynthesis happens in the palisade mesophyll, near the top of the leaf. Roots, stem and leaves together form an organ system that transports substances around the plant.
Xylem carries water and mineral ions from the roots up to the leaves. Water evaporates from the leaves and diffuses out through the stomata. This loss is called transpiration, and it pulls more water up the xylem in a continuous transpiration stream.
Phloem carries dissolved sugars from the leaves to the rest of the plant, for use or storage. This is called translocation, and it can go up or down.
Students say that high humidity speeds up transpiration. Humid air already holds a lot of water vapour, so the concentration gradient is smaller and transpiration is slower.
Answer: 4.8 mm3 per minute.
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