I 20 
Francis Darwin. 
of the vascular system in proportion to the plant, and the number 
of tracheids contained in a stele of given diameter, rather than the 
fundamental type of structure. It is not easy to see how the endarch 
type can be derived from one of the more complicated types of fern- 
stele ; such a hypothesis would appear to be opposed to the evidence 
we possess, and to be, in fact, rather gratuitous. 
Finally it may be recalled that the Hymenophyllaceous frond, 
though in the great majority of cases possessing a main rachis, 
often shows the most marked dichotomy in the branching of its 
pinnae, and this may also be considered a primitive feature, which 
has been retained by many members of the family. 
LECTURES ON THE PHYSIOLOGY OF MOVEMENT 
IN PLANTS. 1 
By Francis Darwin. 
VI.— Diatropism. 
Under this heading I propose to consider both diageotropism 
and dialieliotropism, i.e. the reactions by which plant-organs place 
themselves more or less at right angles to the vertical or to the 
direction of incident light as the case may be. These curvatures 
are identical with pros- and apogeotropism or heliotropism, as far 
as their general character is concerned. The fact that an 
apogeotropic organ is in stable equilibrium when it is vertical, while 
a diageotropic organ’s position of equilibrium is horizontal, is a 
detail which does not interfere with the broad resemblances between 
them. 
According to the statolith-theory a diageotropic organ will be 
at rest when the starch grains rest on the longitudinal walls of the 
statocytes as shown in the accompanying figure; that is to say the 
C D 
A B 
position which calls forth a reaction in a pros-geotropic organ is 
associated with equilibrium in a diageotropic organ. It is a 
remarkable fact that a diageotropic stolon may be rotated on its axis 
1 A Course of Advanced Lectures in Botany given for the Uni¬ 
versity of London at the Chelsea Physic Garden in the 
October term, 1906. 
