I2O 



Elementary Plant Physiology. 



Fit a section of glass tubing 2 cm. in diameter and 10 cm. 

 long with cork stoppers. Perforate one cork with a hole 

 sufficient to receive the base of the main root of a seedling of 

 pea which has a stem a few centimeters in length. Split the 

 cork, and place round the base of the root, and thrust into the 



end of the tube. Fasten 

 the tube in an inverted 

 position, and fill with 

 water. Fit a perforated 

 cork to the upper end of 

 the tube. Note position 

 of roots and stem at be- 

 ginning of experiment and 

 twenty hours later (Fig. 



92). 



130. Progeotropism. 



Germinate a number of 

 seedlings of pea or bean, 

 and when the roots are 1.5 

 cm. in length, take several 

 of them and fasten by means 

 of pins to a sheet of cork 

 or wood, with the tips of 

 the roots directed upward 

 vertically. Float the cork 

 in a dish of water, and 

 cover with some opaque 

 object, such as a board, 



making a moist chamber into which light enters from the 

 sides only. Note the positions of the roots a day and two 

 days later. The roots are seen to react to the stimulation of 

 gravity by a motion which has for its purpose the directing 

 of the roots toward the center of the earth or downward into 

 the soil. 



Fig. 92. Plantlet of pea with root in a 

 small moist chamber, in an inverted posi- 

 tion, to demonstrate geotropic irritability 

 of root and shoot. 



