A plant has no power to select food material. Rather, it is obliged to take up any substance dissolved in the water that is capable of oozing through the vegetable membranes of the root hairs, the tiny threads on each roots. These substances continue to pass through the membrane by osmosis until the liquid on the internal side is as highly charged as that on the outside and then the root stops absorbing. If the plant uses any of these materials for plant nutrition, they are withdrawn from the inner surface of the root up into the plant. At this point, the root is able to take up more of the nutrient in dissolved water from the soil. This system means that normally the passage of useless or injurious substances into the plant is limited. As soon as the internal liquid in contact with the absorbing surface is as highly charged with the undesirable substance as the external liquid, the absorption stops.
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