48 ESSENTIALS OF BOTAXY 



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 diiierence in level there must be a pressure of 

 nearly six pounds per square inch on the stump at the 

 base of the tube T} 



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. 



