16 RICHARDSON—ON TWISTING OF LEAVES 
In some flat-leaved hemlock firs, such as Tsuga Steboldzz, the 
arrangement of the leaves on the horizontal shoots is essentially 
_ the same as that which obtains in flat-leaved silver firs and in 
the Douglas fir; but in such species as Zswga canadensis and Ts. 
Mertenstana, and one or two others, a slight divergence occurs. 
In the leaves inserted in the median plane upon the upper side 
of the shoot, which are generally smaller than the others, no 
twisting on the base takes place. These leaves behave in 
precisely the same way as do those in a corresponding position 
on the horizontal shoots of the flat-leaved spruces, but with this 
difference, that whereas in the spruces the stomata, being on the 
upper side of the leaf, become directed downwards when it bends 
forward in the direction of the apex of the shoot, in these 
hemlock firs, owing to their being on the under side of the leaf, 
they become directed upwards. 
In the flat-leaved spruces, then, in consequence of the stomata 
being located on the upper leaf-surface, the arrangement of the 
leaves on the horizontal shoots is quite distinct from that in 
flat-leaved silver and hemlock firs, and in the Douglas fir. In 
these latter the mode of twisting of the leaves on their bases is 
identical with that observable in a plagiotropous shoot of such a 
plant as the common yew, or in fact of any broad-leaved plant 
such as Deervilla or Philadelphus—that is to say, the direction, 
as seen from above, in which the leaves twist on their bases ona 
horizontal shoot, when they come to occupy positions nearly at 
right angles to its axis, is away from the apex of the shoot, or 
when nearly parallel with it the direction of twisting is away from 
the median plane on the upper side of the shoot. In Figs, 1 and 2 
horizontal shoots of this sort are represented diagrammatically 
as seen from above. The centre line represents the axis, the 
lateral lines the leaves, and the curved arrows show the direction 
in which the leaves twist ori their bases. Fig. 1 illustrates the 
arrangement of the leaves on the horizontal shoots in such flat- 
leaved silver firs as A. grandis and A. Lowtana, and also the 
arrangement on the under side of the shoot in such flat-leaved 
species as A. amadilis and A. Nordmanniana, while Fig. 2 
illustrates the arrangement on the upper side of the shoot in 
species such as the last named. In the case of a flat-leaved 
spruce, on the other hand, the direction in which the leaves 
