i4 RICHARDSON—ON TWISTING OF LEAVES 
surface downwards, and, by a swing movement at its base, - 
which is independent of the twisting movement, it moves 
upwards so that it usually lies in a more or less horizontal 
plane ; and it also moves outwards to a position nearly at a 
right angle to the direction of the shoot. In the leaves arising 
from the shoot on either side of the median plane, more or less 
twisting takes place at the base of each, according to its position 
on the axis, in order to direct its stomatic (upper) surface down- 
wards, the amount through which each twists (assuming the 
direction of the shoot to be quite horizontal, and the median 
plane of the leaf after twisting to be truly vertical) being equal 
to the angular divergence of its point of insertion from that of 
a leaf inserted in the median plane in which no twisting takes 
place—in other words, the twisting commences in the leaves 
adjacent to those in the median plane upon the upper side of the 
shoot and increases as successive leaves are passed through from 
above downwards. By a swing movement at the base, the leaves 
lying on either side of the median plane move upwards or down- 
wards, according to their positions on the axis, so that they 
arrange themselves in a series of superposed more or less 
horizontal planes lying between those of the uppermost and 
undermost leaves of the shoot ; and they also move outwards 
into positions more or less divergent in direction from that of 
the axis, according to their positions thereon, the divergence 
increasing as successive leaves are passed through from above 
downwards from a few degrees in those adjacent to the leaves in 
the median plane upon the upper side of the shoot to nearly a 
right angle in those adjacent to the leaves in the median plane 
upon the under side of it. 
In flat-leaved silver firs, and in the Douglas fir, on the other 
hand, in which the stomatic leaf-surface is morphologically the 
under one, a leaf arising in the median plane upon the upper 
side of a horizontal shoot twists on its base through 180 degrees 
in order to direct its stomatic (under) surface downwards, while 
a leaf arising in the median plane upon the under side of a 
horizontal shoot does not twist, but moves upwards by a swing 
Movement at its base, so that it usually lies in a more or less 
horizontal plane; and it also moves outwards into a position 
nearly at a right angle to the direction of the shoot. As is the 
