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ue 
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THE ROOT 181 
the water absorbed by the roots is taken in by these hairs. They 
contain a lining of protoplasm within which is a large vacuole. 
As a root hair is the most active part of an epidermal cell, the 
nucleus is usually found in the root. hair. 
Since root hairs serve largely for the absorption of water, it 
is not surprising to find that many of the plants ln oTrow 
submerged in water do not possess 
them. 
Physical characteristics of the soil. 
In order to understand how roots 
absorb substances from the soil it will 
be necessary to consider the physical 
characteristics of the soil itself. The 
soil is made up of small, irregularly 
shaped particles of rock and decom- 
posing organic matter, with spaces 
between the particles. If the soil is 
well drained these spaces are largely 
filled with air. The water occurs as 
very thin films which adhere to the 
soil particles. The force by which ics ens Re 
this water is held is very great; it in millet root. (x 280) 
is so great that the water cannot 
all be removed by evaporation without the application of heat. 
The air found between the soil particles is essential to the 
respiration.of the roots of most plants. The presence of air can be 
very clearly demonstrated by putting a mass of soil under water, 
when the water will displace the air in the spaces and the air will 
be seen leaving the top of the soil in the form of bubbles. Many 

Fig. 184. Section showing 
plants cannot live in a soil that is flooded, as such a soil does 
not contain sufficient oxygen for the respiration of their roots. 
Contact of root hairs with soil particles. The formation of 
root hairs aids in the absorption of water not only because it 
increases the absorbing surface but also because the hairs come 
in closer contact with the soil particles than would be possible in 
the case of large roots. 

