THE ROOF ~ 71 
owing to its living in water and its whole surface being 
engaged in the absorption of food material, it does not need 
any tissue for the conduction and transpiration of water and 
tood substances. | 
Xerophytes, on the other hand, often have well-marked 
tap-roots, which penetrate the soil in order to get water that 
is sometimes present at a great depth, for roots are attracted 
by the presence of moisture, and grow towards it. 
