"I feel highly honored, Webb," said his mother, smiling, "that so much science should minister to me and my little collection of plants. I now see that the why and wherefore comes in very usefully. But please tell me why you put the plants that were touched with frost into cold water, and why you will not let the sunlight fall on them?" "For the same reason that you would put your hand in cold water if frost-bitten. Your expression, 'touched with frost,' shows that there is hope for them. If they were thoroughly frozen you would lose them. Your plants, you know, are composed chiefly of water, which fills innumerable little cells formed by the vegetable tissue. If the water in the cells is chilled beyond a certain point, if it becomes solid ice, it expands and breaks down the tissue of the cells, and the structure of the plant is destroyed. If the frost can be gradually withdrawn so as to leave the cells substantially intact, they can eventually resume their functions, and the plant receive no very great injury." "But why does sudden heat or sunlight destroy a frosted plant?" "For the same reason that it breaks down the vegetable tissue. Heat expands, and the greater the heat the more rapid the expansion. When the rays of the sun, which contain a great deal of heat, fall on any part of a frost-bitten plant, that part begins to expand so rapidly and violently that the cellular tissues are ruptured, and life is destroyed. What is more, the heat does not permeate equally and at once the parts affected by frost. The part furthest away from the heat remains contracted, while the parts receiving it expand rapidly and unequally, and this becomes another cause for the breaking up of the vegetable tissue. The same principle is illustrated when we turn up the flame of a lamp suddenly. The glass next to the flame expands so rapidly that the other parts cannot keep pace, and so, as the result of unequal expansion, the chimney goes to pieces. With this principle in mind, we seek to withdraw the frost and to reapply the vivifying heat very gradually and equally to every part, so that the vegetable tissues may be preserved unbroken. This is best done by immersing them in cold water, and then keeping them at a low temperature in a shady place. As the various parts of the plant resume their functions, the light and heat essential to its life and growth can gradually be increased." "It seems to me that your theory is at fault, Webb," said Leonard. "How is it that some plants are able to endure such violent alternations of heat and cold?"