78 RELATION OF PLANTS TO GRAVITATION 



temporarily may be demonstrated in stems of grasses. Cut 

 a length of 12 cm. from a grass stem which will comprise 

 several internodes. Fasten it to a sheet of cork by means of 

 pins in the form of an X at both ends, and a third pair within 

 S cm. of the basal end to give steadiness in the latter part of 

 the test. Place the cork in a horizontal position in a moist cham- 

 ber, or float in a dish of water and cover with a bell-jar. After 

 three or four hours remove the pins that fasten the apical end of 

 the stem and note result. Measure the angle of curvature. 

 Bend a stem of a grass over and pin it to a sheet of cork, 

 or run it through a glass tube just large enough to receive it, 

 and fasten the preparation in a horizontal position. Maintain 

 proper cultural conditions and examine two weeks later. 

 The excitation of the stem by gravity will set the mechanism for 

 curvature in action but the stem can not bend. The altered 

 growth following curvature takes place however, and forms en- 

 largements of the pulvinus, growing out into bulging ex- 

 crescences. 



118. Sensory Zone of Roots. The cells which have the capacity 

 of receiving the geotropic stimulus in the root appear to lie within 

 a millimeter of the tip and embrace the region of undifferentiated 

 tissue. Two lines of experimental evidence tend to this con- 

 clusion. When this portion of the root is cut away the organ 

 ceases to be irritable until regeneration takes place. This test 

 offers insufficient evidence since the shock of the wound might 

 inhibit the normal reaction. A second experiment, exploited by 

 Pfeffer and extended by Czapek is free from errors of this sort 

 and is quite conclusive. The essential part of this demonstration 

 consists in forcing a root to grow into a small section of tubing 

 bent at right angles. When a portion of the root-tip a milli- 

 meter in length passes into the tube and is forced to take a posi- 

 tion at right angles to the main axis, it is found that the geotropic 

 sensibility of the root is determined wholly by the position of 

 this terminal portion. If the terminal portion is allowed to re- 

 main in the tube in a horizontal position it will elongate until it 



