ii6 



Elementary Plant Physiology. 



Fig. 85. 



-Positions assumed by leaves of sunflower when placed horizontally. The 

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



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. 



