various tissues of the plant are under tensions which may exhibit differences equal to 12 atmospheres or more. What is termed the "shearing stress" often becomes so great that the resistant cell walls are ruptured, a condition associated with great pressure in living cells. The injection of poisons into trees may likewise cause a rupturing of
the tissues owing to changes in the turgescence of the cells, and the splitting of melons in the field sometimes occurs from the absorption of an excessive amount of water into the inner cavity of the fruit. This increases the turgescence of the cells lining the cavity, and modifies the existing tissue tensions. The skin of the grape often cracks, possibly from the same cause.
It has been shown that the mechanical restriction of growth acts as a stimulus, inducing an increase in the osmotic pressure of the living cells, and in like manner, increased tensions may result in a much greater strength of the organism. It has been observed, for example, by Hegler, that a young sunflower seedling having an original breaking stress of 160 grams was able to maintain 250 grams after it had been stretched by a weight of 150 grams for two days, and later stretching of the stem by means of suspended weights over a pulley demonstrated that in a few days more its tensile strength was increased to 400 grams. This increase is correlated with thickness of the cell walls, a greater elasticity and the development of mechanical tissue.
The stimulation induced by the contact of tendrils and hook plants with objects is similar to that caused by stretching by weights. Experiments with the roots of various plants enclosed in plaster casts have shown large pressures. Pfeffer obtained osmotic pressures in the root cells of a common horse bean ranging from 5 to 19 atmospheres when the growth of the roots in length (longitudinal pressure) was
