IN SPRUCES AND FIRS, 19 
a pseudo-distichous arrangement is impossible. The leaves in 
the median plane upon the upper side of a horizontal shoot do 
not twist on their bases, nor do they move to either side of the 
shoot, while those adjacent to them on either side twist towards, 
not away from, the median plane, as viewed from above, so that 
there is no parting or shedding along the upper side of the 
shoot, and therefore no pseudo-distichous arrangement.! 
Figures 6-10 will serve to illustrate the various points dealt 
with in the preceding pages, and they will also serve to show 
how the positions of the tissues of the leaves are affected from 
a morphological point of view by the twisting and other move- 
ments which take place at the leaf base, a matter in regard 
to which some misconception seems to exist in the descriptive 
accounts of some of the flat-leaved species of Picea. — 
Fig. 6 represents diagrammatically on a ground plan the 
positions assumed by the leaves in an erect (orthotropous) shoot 
of a flat-leaved silver or hemlock fir, or of the Douglas fir, while 
Fig. 8 represents the same thing in a flat-leaved spruce. In the 
figures the axis of the shoot occupies the centre, and the leaves 
are arranged in a circle surrounding it, the spiral arrangement 
being disregarded in order not to introduce complications. The 
number of leaves (twelve) fixed upon is purely arbitrary, the even 
number being adopted in order to avoid fractions of a degree. 
The leaves are numbered consecutively, and the angular 
divergence from zero (leaf 1) is indicated on the outside of each 
leaf. The various tissue-groups of the leaf are indicated thus : 
—X = xylem; P = phloem; R.C. = resin-canals ; $.S. = stomatic 
surface. 
A glance at Figs. 6 and 8 will show that they differ in one 
particular only—namely, the position of the stomatic surface of 
the leaf. In Fig. 6 it is in the normal position on the phloem- 
side of the leaf, but in Fig. 8 it ison the xylem-side and faces 
the axis of the shoot. 
1 The arrangement of the leaves on the horizontal shoots of as (flat-leaved 
oe other) is frequently incorrectly described as pseudo-distichous. Dr. Engelmann, 
in Watson’s Flora of California, II, p. 121, describes the leaves of Picea as oo? 
ed all round the branchlets or (by a twist of the base) somewhat 2-ranked,” and 
other authorities orggeeed refer to them as being ‘‘2-ranked,” ‘‘2-rowed,” or 
** pseudo-distichou 
C 
