Ce RICHARDSON—ON TWISTING OF LEAVES 
The curved arrows on the central axis indicate 
the direction in which it twists in each successive 
—- internode. 
In the Irish yew all the shoots are ortho- 
Ls tropous and the leaf-arrangement is polystichous, 
\ being in fact a % spiral arrangement. In 
—+—+—}-_ the common yew, of which the Irish yew is only 
a variety, most of the shoots are plagiotropous, 
{ and the leaves, although really spirally arranged, 
become pseudo-distichous by twisting and 
a oes BEE swing movements on their bases, but here there 
a is no torsion of the axis as in Dzervilla and 
Philadelphus. 
In flat-leaved silver firs, and in the Douglas 
Fig. & fir, there is a pseudo-distichous arrangement of 
_... the leaves on the horizontal shoots which, as 
ing of shoot and before mentioned, is identical with that which 
leaves in Dzervil/a. F ; 
occurs in common yew. In such species as 
Altes grandis asnd A. Lowiana this pseudo-distichous arrange- 
ment of the leaves is brought about independently of the twisting 
of the leaves on their bases by the way in which they move out- 
wards on either side of the shoot into positions nearly at right 
angles to the direction ofits axis. In species like A. amadz/zs and 
A. Nordmanniana the pseudo-distichous arrangement is often 
masked by the upper leaves assuming directions parallel with, 
or only slightly divergent from, that of the axis. But, as the 
direction in which the leaves twist on their bases on the upper 
side of the shoot is away from the median plane, as viewed from 
above, their stomatic (under) surfaces turn outwards from each 
other in opposite directions, to either side of the shoot, so that 
there is a parting or shedding of the leaves along the median 
plane on the upper side; and as there is also a parting or 
shedding of the leaves by the swing movement already referred 
to along the median plane on the under side of the shoot, a 
pseudo-distichous arrangement is the result. The resemblance 
between the arrangement on the upper sides of the horizontal 
shoots here and that of the flat-leaved spruces is therefore 
entirely superficial. In a flat-leaved spruce, on the other hand, 
