48 ESSENTIALS OF BOTANY 
stem through which the water from the soil rises freely. 
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. 23 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 sap-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 7.1 
A black-birch root tested in this way at the end of 
April has given a root-pressure of thirty-seven pounds to 
the square inch. This would sustain a column of water 
about eighty-six feet high. 
1 See Appendix II. 
