132 PLANT PHYSIOLOGY 



and inward at the same angle. If the wheel is rotated in a 

 vertical plane, the effect of gravity is entirely eliminated, for 

 it acts on all sides in succession, and it is only the centrifugal 

 force that comes into play. (Such an apparatus with the wheel 

 rotating in the vertical plane can be con- 

 structed by using a stout knitting needle for 

 an axis, the bearings being little cups of glass 

 made by sealing and cutting off short the end 

 of a glass tube. These are inserted into corks, 

 fastened to two upright supports. At the 

 center of the knitting needle is placed a large 

 cork with short knitting needles radiating in 

 ^ our directions in a plane at right angles to 

 the main axle. The ends of these hold corks, 

 which are connected to each other by a wire, 

 which forms the circumference of the wheel. On this wire are 

 strung a number of small cork disks. A stream of water is 

 directed at these disks, and causes the wheel to rotate at a high 

 speed. Seedlings to be experimented with are pinned firmly 

 to the cork disks.) 



(n) Make a thin section of a root cap of a growing root, 

 stain with iodine to make the starch grains more easily visible, 

 The cells containing them are supposed by some botanists to 

 be the perceptive cells for gravity (statocysts). 



(o) On a vigorous plant of cucumber or squash or pea, make 

 the following experiment with the tendrils. Place a very 

 smooth glass rod in contact with one tendril, and a rough stick 

 of the same diameter in contact with another equally developed 

 one. Note the time in each case before the first curvature is 

 noticeable and until the tendril has made one complete turn 

 around the object. Note when the formation of the coils 

 between the object and point of attachment of the tendril 

 first begins, and observe how a twisting of the tendril is avoided 

 as these coils develop. 



(p) Wet a piece of filter paper with Sachs' culture solution 

 and sow on it fresh pollen grains of various kinds, keeping the 

 different kinds on different parts of the paper, but all at about 

 the same distance from the center. Cover to prevent evapo- 

 ration. After a few hours, examine and if germination has 

 occurred, place a small crystal of cane sugar at the center. 

 Examine every two or three hours, and note when and where 



