CURVATURE OF SHOOTS 



83 



126. Region and Form of Curva- 

 ture of Shoots. Grow a number of 

 seedlings of Helianthus. When a few 

 centimeters high take them from 

 the sawdust or sand in which they 

 have been grown, and fix a half dozen 

 by means of split corks in bottles of 

 water. Mark the stem of each into 

 centimeter intervals by means of a 

 ruler, and a thread saturated with 

 India ink and held taut by a pair of 

 calipers. Now lay the bottles on their 

 sides in a damp chamber at a tem- 

 perature of about 20 C. in diffuse 

 light for three hours. Note the region 

 in which curvature occurs. Allow the 

 curvatures to proceed. It will be 

 seen that the tip of the shoot will 

 be carried upward and past the ver- 

 tical and then back again (See Fig. 



37)- 



127. Mechanism of Curvature of 



Grass Stems. Cut a few rapidly 

 growing stems of some convenient 

 grass and mark 2 mm. intervals on 

 opposite sides of some of the nodes. 

 Now place these stems in a horizontal 

 position, with one end in a heap of 

 moist sand and cover the whole with 

 a glass vessel to make a moist cham- 

 ber. Take out the stems a day later 

 and measure the distance between the 

 marks on the upper and lower sur- 

 faces of the nodes. Note that the 

 hairs on the lower surface are diverg- 



FlG. 37. Apogeotropic curva- 

 ture of a shoot. I, horizontal posi- 

 tion of stem at beginning of ex- 

 periment. z-15, successive stages 

 in upward curvature. 16, final 

 position of shoot. After Sachs. 



