212 TEXTBOOK OF BOTANY 



upon the concentration of water inside and outside the cell, the conse- 

 quent osmosis and cell turgor, and the extension of the cell wall. 



In some unknown way a hormone ( auxin ) formed in the young leaves 

 seems to be necessary to some process involved in the extension of cell 

 walls. If the hormone is absent little or no cell wall extension occurs. 

 If it varies in amount in different parts of the plant, the cells do not 

 enlarge uniformly. It is perhaps evident that turgor pressure will not 

 become so high in the cells with walls that are growing or are easily 

 extended. We may therefore have the anomalous condition of the en- 

 trance of water as a cause of increase in cell volume, and at the same 

 time the lowest turgor pressure in the enlarging cells. If the cell walls 

 are not growing or do not stretch readily, the entering water may result 

 in a high pressure within the cells. 



Turgor and growth pressure. Similarly, if the enlargement of a plant 

 organ is restricted by mechanical means, the diffusion of water into each 



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Fig. 72. Fern leaves pushing upward through a cement sidewalk. Growth pressure 

 may amount to hundreds of pounds to the square inch. After G. E. Stone. 



of its cells exerts tremendous force against the obstruction (Fig. 72). 

 The combined pressure in all the cells of a root may lift stones weighing 

 several tons, displace stone curbings, or rupture concrete pavements. 

 Turgor pressure may be just as great in the cells of stems and other plant 

 organs. It is the force underlying the pushing of young stems of seed- 

 lings upward through the soil and of roots into the soil. 



Since turgor pressure in cells is the result of the diffusion of water 

 into them, the energy of molecular motion is evidently the force back 

 of all these "marvelous powers" of plant growth. While the pressure 

 exerted by the plant organ as a whole may be equivalent to several 

 hundred pounds per square inch, a little calculation will show that the 

 pressure on a single cell wall is but a small fraction of an ounce. This 

 pressure is usually great enough to cause the walls of cells to bulge out- 

 ward and become convex except where they press against adjacent cells 

 and the surfaces become flattened or plane. The same phenomena may 



