TEE POWER OF GROWTH IN PLANTS 



235 



come the resistance offered by the concrete. This we consider a very 

 fair estimate, although from our other experiments we are led to believe 

 that as high as 50 atmospheres are sometimes required to accomplish 

 the work with the conditions under which the ferns were growing. 

 The concrete was so hard that after it had been ruptured it was impos- 

 sible to make any impression. on the ragged edges except by the use of 

 tools. The work was done by a slow and constantly increasing pres- 

 sure on the under surface of the concrete, the principle being somewhat 

 the same as in the straightening of teeth and bones, although in such 

 cases the pressure is not increased. 



Fig. 6. Method of determining the longitudinal power of growth in roots. The 

 roots are held firmly in two plaster of Paris casts, and the amount of pressure indi- 

 cated by the spring. (After Pfeffer.) 



At this point we might consider what growth is and how it is ac- 

 complished in a plant. Growth is defined as a stretching and fixation 

 of the cell walls, accomplished by osmotic pressure characteristic of the 

 solutions contained in the cell vacuole. In ordinary growth there is a 

 pressure of 1 to 3 atmospheres on the cell walls — a fact which can be 

 determined experimentally with some degree of accuracy. It is this 

 pressure which gives plants their rigidity and freshness, and anything 

 which destroys it, such as lack of water, causes the plant to wilt. 

 Eapidly growing organisms — annuals and herbaceous plants, for in- 

 stance^ — contain little mechanical or supportive tissue, and it is owing 

 to the turgidity of the cells derived from osmotic pressure that they 



