8 



EXPERIMENTAL PLANT PHYSIOLOGY. 



the root-hairs (Fig. 3). The amount of surface extension ob- 

 tained by root-hairs is very great, since these structures are 

 .008 mm. to .14 mm. in diameter, and often attain a length of 

 3 mm., while from 10 to 400 may be formed on a square 

 millimeter of surface. 



The root-hairs are also an adaptation for obtaining water 

 FIG. 4. under the conditions in which it is 



found in the soil, where it occurs in the 

 form of a minute layer on the surface 

 of the soil-particles. The root-hairs 

 are capable of bending around and 

 penetrating between the particles in a 

 manner which places their walls in con- 

 tact with a large amount of this layer 

 of water. In aquatic plants root-hairs 

 are not needed, and are rarely formed, 

 since the entire body of the root is in 

 contact with the water. It will be seen 

 that the land plants grown in water in 

 the culture experiments developed very 

 few root-hairs. On the other hand, 

 plants grown in dry soil exhibit a very 

 marked development of these struc- 

 tures. In this instance the amount of 

 Seedlings of White Mus- water around each soil-particle is very 



tard. (Sachs.) A, with 



soil clinging to the roots ; small, and the plant must reach a 



B. after removal of the . . 



mass of soil by washing, much larger number of them in order 

 to obtain the needed supply. 



EXPERIMENT 5. 



ADHESION OF ROOT-HAIRS TO SOIL-PARTICLES. 



Grow seedlings of Mustard, Pea, or Corn in sandy soil. When 

 one week old take up and note amount of soil clinging to roots 

 (Fig. 4). Free from the mass of the soil by washing. Examine 



