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 impossible to make any impression on the ragged edges except by the use of tools. The work was done by a slow and constantly increasing pressure 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 indicated by the spring. (After Pfeffer.)
At this point we might consider what growth is and how it is accomplished 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. Rapidly growing organisms—annuals and herbaceous plants, for instance—contain little mechanical or supportive tissue, and it is owing to the turgidity of the cells derived from osmotic pressure that they
