54 FOUNDATIONS OF BOTANY 
possessed of intelligence and will. Plants of two different 
species, both growing in the same soil, usually take from 
it very various amounts or kinds of mineral matter. For 
instance, barley plants in flower and red-clover plants in 
flower contain about the same proportion of mineral mat- 
ter (left as ashes after burning). But the clover contains 
52 times as much lime as the barley, and the latter contains 
about eighteen times as much silica as the clover. This 
difference must be due to the selective action of the proto- 
plasm in the absorbing cells of the roots. Traveling by 
osmotic action from cell to cell, a current of water derived 
from the root-hairs is forced up through the roots and into 
the stem, just as the contents of the egg was forced up 
into the tube shown in Fig. 24. 
66. Root-Pressure. — The force with which the upward- 
flowing current of water presses may be estimated by 
attaching a mercury gauge to the root of a tree or the 
stem of a small sapling. This is best done in early spring 
after the thawing of the ground, but before the leaves 
have appeared. The experiment may also be performed 
indoors upon almost any plant with a moderately firm 
stem, through which the water from the soil rises freely. 
A dahlia plant or a tomato plant answers well, though the 
root-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. 25 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 root-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 See Handbook. 
