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 
oa. cork with short knitting needles radiating in 
Fic. 58.—Geo- four directions in a plane at right angles to 
ment (m). 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). 
(0) 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 
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