DETERMINATION OF REACTION TIME 



75 



ceived by orthotropic organs at a divergence of 135 from the 

 normal, and plagiotropic organs at 90 from the normal. 1 



In the following experiments the term progeotropism will be 

 used to denote the form of reaction which places the apex of 

 the organ in a vertical position 

 and pointing toward the center 

 of the earth. Apogeotropism 

 designates the reverse action, 

 and diageotopism denotes the 

 action of organs which place 

 their axes at right angles to the 

 line of gravity (See also lateral 

 geotropism). 



112. Determination of Reac- 

 tion Time. Place a number of 

 seeds of Avena sativa, Phalaris, 

 or Beta vulgaris in a moist cham- 



FlG. 33. Plate of cork or cube of turf 



ber, or in damp sawdust until attached to extension arm of clinostat. 

 germination takes place. Fasten Seedlings are fastened to the cork or turf 



these seedlings in a plate of and the ori s inal P sitions of the shoots 



. . .. . , . and roots marked by pins. After Sachs. 



cork by means of a pin driven 



at an acute angle through their main axes, with the tips of the 

 roots directed upward at an angle of 135, and the hypocotyls 

 downward at the same angle. Float the cork in a shallow dish, 

 and cover with a bell-jar in such manner that a horizontal micro- 

 scope may be focussed on the tip of a root. During the first 

 few minutes a downward sagging movement due to the weight 

 of the plastic roots may be observed, and should not be confused 

 with irritable reactions. Adjust the instrument to take up this 

 motion, and keep the tip of the root on the cross-hair of the 

 instrument. Twenty-five minutes later keep under continuous 

 observation and note exact moment when downward movement 

 begins. The temperature should be maintained at 18 to 25 C. 



i Czapek, F. Weitere Beitrage zur Kenntniss der geotropischen Reizbebewegun- 

 gen. Jahrb. Wiss. Bot. 32: 175. 1898. 



