IN SPRUCES AND FIrRs. 17 
twist is, when viewed from above, either towards the apex of 
the shoot, as represented in Fig. 3, which illustrates the 
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Fig. J Fig. 2 Fig. 3 Fig. 
Figs. 1 and 2. Directions of twisting and movement in silver fir. 
Figs. 3 and 4. Directions of twisting and movement in spruce. 
arrangement of the leaves on the under side of the shoot, or 
towards the median plane on the upper side of the shoot, as 
represented in Fig. 4, which illustrates the arrangement on the 
upper side of the shoot. 
In plagiotropous shoots in which the stomatic leaf-surface is 
morphologically the under one, and where the leaf-arrangement 
is normally not a truly distichous but a polystichous one, a 
pseudo-distichous arrangement is frequently brought about 
either by twisting and other movements of the leaves on their 
bases, or by torsion of the axis itself. In Dvzervilla and 
Philadelphus the leaves are opposite and decussate on the 
orthotropous shoots, but they all lie in one horizontal plane on 
the plagiotropous shoots, with their stomatic surfaces directed 
downwards. In these cases the pseudo-distichous arrangement 
on the plagiotropous shoots is brought about by torsion of the 
axis alternately to right and left between the nodes, so as to 
bring alJl the points of insertion of the leaves into nearly the 
Same plane, and at the same time each leaf twists at its 
base through go degrees and brings its surface into a horizontal 
position, so that they all lie in the same horizontal plane. This 
arrangement is illustrated diagrammatically in Fig. 5, which 
represents a plagiotropous shoot of this sort as viewed from above. 
