If you will turn back for a moment to the beginning of the description of how plants get their food, you will find that in osmosis the weaker liquid tends to permeate the denser one more rapidly than the denser one does the weaker. As we have just seen, the sugary liquid in the root hairs is denser than the soil water outside, and, furthermore, _none of it is allowed to escape_. This comparatively greedy process of taking everything and giving nothing results in a constant flow of soil water into the root hairs. When the flow of liquids in osmosis is not at once equalized, a gentle pressure is brought to bear to make them so. This is what is called _osmotic pressure_, and it is this pressure that forces the absorbed liquid through the roots and part way up the trunk of even the tallest trees. While we have just said it is a gentle pressure, that is true only in the case where the osmosis has free play, and the pressure is stopped with the perfect mixing of the two liquids. But what if they can never mix? What may not the accumulated osmotic pressure amount to in such a one-sided process as goes on in root hairs with everything coming in and nothing going out. Cut-off stems, with a pressure gauge attached to them, indicate that in some plants the pressure is from 60 up to 170 pounds! Another result of this pressure is that it keeps leaves and the fleshy stems of plants in their ordinary position. The actual solid part of nearly all leaves is scarcely 5 per cent of their bulk and all the rest is water. The constant pressure of this water from the roots is sufficient to keep leaves comparatively stiff and rigid, how stiff is quickly realized if the pressure stops and the leaf wilts or withers. Sometimes this osmotic pressure, particularly during rainy weather, becomes so great as to cause injury to the plant, the splitting of tomatoes and occasionally of plums, being due to it. This osmotic pressure, together with the extra pull given by the leaves, is sufficient to account for the rise of water to the tops of the tallest trees. The tallest trees in the world are certain kinds of blue gum in Australia which frequently reach a height exceeding 300 feet. What the combined osmotic pressure and leaf pull must be to carry such a heavy thing as water to such a great height is easier to imagine than to calculate.