30 HOOTS 



A dahlia plant or a tomato plant answers well, though the 

 sap pressure from one of these will not be nearly as great as that 

 from a larger shrub or a tree growing out of doors. In Fig. 24 

 the apparatus is shown attached to the stem of a dahlia. The 

 difference of level of the mercury in the bent tube serves to 

 measure the pressure. For every foot of difference in level there 

 must be a pressure of nearly six pounds per square inch on the 

 stump at the base of the tube T. 1 



A black birch root tested at the end of April has given a 

 sap pressure of thirty-seven pounds to the square inch. This 

 would sustain a column of water about eighty-six feet high. 



39. Root absorption and temperature of the soil. The tem- 

 perature of roots and the earth about them has much to do 

 with the rate at which they absorb water. Some plants can 

 absorb it at temperatures as low as 25 F. ( 4 C.), while 

 others cannot do so at any temperature below 39 F. (4 C.). 

 This fact of the power to get water from the soil ceasing at tem- 

 peratures in the neighborhood of the freezing point has most 

 important consequences, since it implies that a plant may die 

 for lack of water with its roots immersed in cold, wet soil. 

 Hence the parched appearance often noticed in leaves killed 

 by frost. 



40. Movements of young roots. The fact that roots usually 

 grow downward is so familiar that we do not generally think of 

 it as a thing that needs discussion or explanation. Since they 

 are pretty flexible, it may seem as though young and slender 

 roots merely hung down by their own weight, like so many bits 

 of wet cotton twine. But the root of a young Windsor bean 

 seedling or of a sprouting pea will force itself down into mer- 

 cury. By comparing the weights of equal bulks of mercury and 

 Windsor bean roots, it is found that the mercury is about four- 

 teen times as heavy as the substance of the roots. Evidently, 

 then, the submerged part of the root must have been held under 

 by a force about fourteen times its own weight. 



1 For a more accurate method see Handbook. 



