THE POWER OF GROWTH IN PLANTS 



237 



various tissues of the plant are under tensions which may exhibit dif- 

 ferences equal to 12 atmospheres or more. What is termed the " shear- 

 ing 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 



Fig. 8. 



Showing squash in harness. A weight of 5,000 pounds was lifted. 

 (After Clark.) 



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, pos- 

 sibly 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. Pfefier 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 



