zo8 
MORPHOLOGY OF MEMBERS. 
mother-shoot, and at the same time to the direction of gravitation, light, and pressure 
(the latter in clinging or climbing plants, such as ivy, Jungermannieae, &c.). It is 
therefore probable that internal and external causes generally cooperate to determine 
the direction of the longitudinal axis of a member when first formed, as well as of its 
lateral shoots Further development may show new relations to the original axis and 
to external influences. The horizontal lateral shoots of numerous woody Dicotyledons 
with alternate leaves in two rows have the principal section vertical, their rows of leaves 
right and left. The axillary buds of these leaves which remain dormant through 
the winter show an altogether different disposition of 
their parts ; the axis of the bud is parallel to that of 
the mother-shoot ; it bears its leaves in two rows, one 
facing the sky and the other the earth (Fig. 155) ; the 
mid-ribs of the folded leaves are always turned out- 
wards, away from the mother-axis; the principal axis 
of the whole bilateral shoot (the bud) is horizontal. 
But when the bud unfolds in the spring, a torsion 
takes place of such a nature that the principal section 
assumes a vertical position, the prominent mid-ribs of 
the leaves turn downwards, while their faces turn up- 
wards ; and thus the lateral shoot of a horizontal mother- 
shoot acquires the same position as its parent. The 
fact that the two rows of leaves within the lateral 
bud arise on the upper and under side, and consequently 
both in a vertical plane, might be referred to the im- 
mediate influence of gravitation ; but this view is 
opposed by the fact, among others, that the position of 
the terminal bud ^ of the horizontal mother-shoot is 
usually from the first different. In Cercis and Corylus, 
for example, the terminal bud stands on the under 
side of the horizontal end of the branch, and its leaves 
right and left of the vertical principal section of the 
bud. The position of a terminal bud may be easily 
represented by turning Fig. 155 so that the parent axis 
a lies above, the subtending leaf b beneath the bud (the apparently terminal bud thus 
becoming axillary), and the vertical line v becomes horizontal. This difference, which 
exists from the very first in the position of the lateral and terminal buds in horizontal 
bilateral mother-shoots, excludes the immediate influence of gravitation ; but by a useful 
adaptation, even in the bud all the parts are so arranged that by a single twist of the axis 
during unfolding, they assume those positions which are most favourable for the functions 
of the leaves, and by which their faces are turned towards the light. In the terminal 
buds of such shoots this twisting is not necessary. Whether it is gravitation or the 
influence of light on growth which occasions this torsion of the axis of the bud is not 
Fig. 153.— Lateral bud of a horizontal branch 
of Cercis canadensis (in December), in vertical 
transverse section ; 1—7 the consecutive leaves 
with their pairs of stipules indicated by the same 
numbers. The outer bud-scales have been re- 
moved, the two inner ones indicated by 3, 3. In 
the centre is the growing- point of the bud. b the 
position of the leaf in whose axil the bud grows ; 
a axis of the mother-shoot ; v direction of gravi- 
tation. 
point of view ; but on consulting the facts themselves I find much that is not in agreement with his 
statements, and in their interpretation 1 arrive at essentially different conclusions, the reasons for 
which cannot be explained here in detail. 
^ In order to determine this question experimentally, the apparatus which is described in Book 
III. Chap. 3, Sect. lo, should be used, a drum which holds the plant, and which rotates slowly 
round a horizontal axis. 
^ It is for our present purpose the same whether the bud at the end of the horizontal shoot be 
its true terminal bud, or a lateral bud the development of which is induced by the abortion of a 
terminal bud, as in Cercis and Coryhis. In reference to the position of the terminal bud it is also 
indifferent that the position of the lateral buds is sometimes not quite horizontal, l)ut a little 
oblique upwards and outwards, as in Corylus, Celtis, Sec. 
