478 IOWA ACADEMY OF SCIENCE Vol. XXVI, 1919 



expended by growing plants and their roots, is almost incredibly 

 strong. A recent textbook- gives an illustration of a millstone ten 

 and one-half inches thick cracked in two by an elm tree in thirteen 

 years after it had sprouted through the central opening. 



Some idea of the shoving power in growing plants may be had 

 from a few illustrations of cell pressure. "The root of the garden 

 pea, for instance, has a wedging force equal to 200 or 300 pounds a 

 square inch."^ I am informed by Dr. R. B. Wylie, of our State 

 University, that the osmotic pressure of the plant cell of a sugar beet 

 is as high as twenty-four atmospheres. Concerning the action of 

 roots Doctor Stockbridge, a careful student of their effect upon 

 rocks and soils, says, "roots permeate the rock-mass wherever the 

 slightest crevice offers an entrance ; and then, by the expansive force 

 of the growing tissue, the most tenacious of rocks are rent and torn 

 asunder, no power in nature being able to withstand the force of 

 this slow-working but resistless expansive action." And again, 

 speaking of the power of the root system in forcing the soil-water 

 upward, he says, "the pressure exerted by a birch-root severed from 

 its connection with the tree was equal to a column of water 85 feet 

 in height ; and that of a squash-plant eight weeks old, soft, open in 

 its texture, and very tender, exerted a force equal to a column of 

 water 45.5 feet high."'* 



During the winters in our latitude living tree trunks contract at 

 times of very low temperatures and expand again as the thermom- 

 eter rises. At times frost cracks appear and disappear in some 

 trunks under these conditions. The actual volumetric decrement or 

 increment is of low amplitude but the force is inexorable and no 

 doubt plays a small part in the wedge-work of the larger roots near 

 the surface and of the base of the trunk when the latter is ob- 

 structed. The reader is referred to some interesting observations 

 on "The Coefficient of Expansion of Living Tree Trunks" which ap- 

 peared in a late number of Science.^ 



In discussing the agents of weathering textbooks of physiography 

 and general geology in many cases give illustrations of rocks or 

 boulders which have been cleft by the wedge-work of roots — the 

 cuts usually showing a tree growing out of the cleft. Boulders, 

 however, exhibiting this feature are not common. In spite of the 

 fact that glacial boulders are numerous and widely distributed in 



=Ries and Watson, Engineering- Geolog^y, New York, 1914, PL XXXIII, 

 fig. 2. 



'Cleland, H. F., Physical Geology, Chicago, 1916, page 33. 



♦Stockbridge, H. E., Rocks and Soils, 2nd Edit., New York, 1906, pp. Ill, 112, 

 and 184. 



^Trowbridge and Weil, Science, new series. Vol. XLVIII, Oct. 4, 1918, pp. 

 348-350. 



