CLIMBING PLANTS. 407 
row ellipses. After performing thirty-seven revolutions the 
stem of a hop was found to be twisted three times round its 
own axis in the direction of the sun. To prove that the 
twisting of the stem does not cause the revolutions, as Hugo 
von Mohl supposed, some stems are not regularly twisted 
and others twist in an opposite direction to the revolving 
plant. In many twining plants the end of the shoot is 
hooked so as the more readily to hold fast to any object of 
support which may be caught. This support once found, 
the point of contact ceases to move, but the tip continues to 
twine above and around the support as a rope swung around 
a stick will coil in the direction of the swinging rope. 
If a stick shortly after having been wound round be with- 
drawn, the shoot retains for a time its spiral form, then 
straightens itself and again begins to revolve. Mohl be- 
lieved that plants twined because of a dull irritability of the 
stem, but experiments prove that this is not generally the 
case. 
If the support of a twiner be not lofty it falls to the 
ground, and resting there the extremity rises again. Some- 
times several flexible shoots twine together into a cable and 
thus support each other. Single thin shoots will fall and 
turn abruptly back and wind upwards on themselves. The 
majority of twiners move in a course opposed to that of the 
sun or the hands of a watch. Rarely plants of the same 
order twine in opposite directions, but no instance is known 
of two species of the same genus twining in opposite direc- 
tions. Of seventeen plants of. Loasa aurantiaca, eight re- 
volved in opposition to the sun and ascended from left to 
right, five followed the sun and ascended from right to left, 
and four revolved and twined first in one direction, and then 
reversed their course. One of these four plants made seven 
spiral turns from right to left, and five turns from left to 
right. Climbers of the temperate zone will not generally 
twine around thick trees, while those of the tropics can. 
Unless this were the case those of the tropics could hardly 
