22 RICHARDSON—ON TWISTING OF LEAVES 
to this twisting there is the swing movement at the base, by 
which the leaf moves upwards and outwards into the position 
indicated in the figure ; and, as the point of insertion of this leaf 
is in the median plane upon the under side of the shoot, it may 
move either to the right or to the left. In the leaves lying 
between 1 and 7, on either side of the median plane of the shoot, 
the same rule as to twisting obtains as that which governs the 
twisting in Fig. 7, but here the order of succession in which 
the leaves twist is reversed in direction as compared with that 
illustrated in Fig. 7 
The curved arrows beneath Fig. 8 indicate the direction in 
which the leaves shed away from the median plane of the axis 
when a shoot such as this becomes horizontal as in Fig. 9; 
but the shedding of the leaves along the median plane on the 
under side of the shoot is not here due to a swing move- 
ment at the base only, as in Fig. 7, but to a combination of 
both a twisting and a swing movement. Both these movements, 
in fact, culminate in the leaves in the median plane on the under 
side of the shoot in a flat-leaved spruce ; whereas in a flat-leaved 
silver, in some hemlock firs, or in the Douglas fir, the twisting 
movement culminates in the leaves in the median plane on the 
upper side of the shoot, while the swing movement culminates in 
those in the median plane on the under side. 
Fig. 10 represents, in the same way as in Figs. 7 and 9, the 
positions assumed by the leaves on a horizontal shoot of a hem- 
lock fir such as 7suga canadensts, or T. Mertenstana, as described 
on p. 16. The leaves inserted in the median plane upon the 
upper side of the shoot show no twisting at the base, but, 
bending forward in the direction of the apex of the shoot, 
they occupy positions similar to that of leaf 1 in Fig. 10, 
in which the stomatic (under) surface is directed upwards, 
whereas in all the other leaves of the shoot it is directed down- 
wards as in the flat-leaved silver firs, and in the Douglas fir.} 
1 Ina paper entitled a “ Review of some Points in the Comparative Morphology, 
Anatomy, and Life-History of the Coniferze,” published in the ‘‘ Journal of the Linnean 
ociety, Botany, Vol. xxvii, Dr. Masters refers to the leaf-arrangement in these 
plants as follows (p. 247) :—‘‘ Another instance of variation in the arrangement of 
“leaves is often seen in Abies Nordmanniana, A. Pichta, A. amabilis, as also in 
Tsuga canadensis, &c. The leaves on the lateral and —_ or a hereon 
** spreading Denlinlies, though polystichous, i 
7 = 4! 
