n6 



Elementary Plant Physiology. 



Fig. 85. Positions assumed by leaves of sunflower when placed horizontally. The 

 blades are perpendicular to the rays of light. After Dels. 



and twist the end, or tie around it with a thread. Cover two 

 of the seedlings with these caps in such a manner as to totally 

 exclude the light. Cover two more with 

 short caps extending downward a distance of 

 6 to 8 mm. only from the tip. Slip cylin- 

 ders over two more in such manner that the 

 basal portion only is covered, leaving a length 

 of a centimeter at the tip exposed. Set the 

 preparation in a dark chamber, as described 

 above, leaving the aperture open at the level 

 of the seedlings. Note positions a day or 

 two later. (See " Practical Plant Physi- 

 ology.") What portion of the leaf must 

 light strike to set up a reaction? 



126. Motor zone in Coleus Select a 

 rapidly growing specimen of Coleus, and strip 

 it of the leaves. Wrap the stem with 

 sphagnum, or cover with tin-foil to within a 

 centimeter of the tip, or leave one inter- 

 node free. Place in a dark chamber arranged 

 as above. Two days later note the region 



Fig. 86. Dia- 

 gram of stem of 

 Coleus, to show 

 region of curva- 

 ture when illu- 

 minated at tip, 

 above horizontal 

 line. After 

 Rothert. 



