ssumption that
those plants have survived which have acquired through unknown causes
the power of reacting in appropriate ways to the external stimuli of
light, gravity, etc. It is quite possible to conceive this occurring in
plants which have no power of circumnutating--and, as already pointed
out, physiologists do as a fact neglect circumnutation as a factor in
the evolution of movements. Whatever may be the fate of Darwin's theory
of circumnutation there is no doubt that the research he carried out
in support of, and by the light of, this hypothesis has had a powerful
influence in guiding the modern theories of the behaviour of plants.
Pfeffer ("The Physiology of Plants", Eng. Tr. III. page 11.), who more
than any one man has impressed on the world a rational view of the
reactions of plants, has acknowledged in generous words the great value
of Darwin's work in the same direction. The older view was that, for
instance, curvature towards the light is the direct mechanical result of
the difference of illumination on the lighted and shaded surfaces of the
plant. This has been proved to be an incorrect explanation of the
fact, and Darwin by his work on the transmission of stimuli has greatly
contributed to the current belief that stimuli act indirectly. Thus we
now believe that in a root and a stem the mechanism for the perception
of gravitation is identical, but the resulting movements are different
because the motor-irritabilities are dissimilar in the two cases. We
must come back, in fact, to Darwin's comparison of plants to animals.
In both there is perceptive machinery by which they are made delicately
alive to their environment, in both the existing survivors are those
whose internal constitution has enabled them to respond in a beneficial
way to the disturbance originating in their sense-organs.

XX. THE BIOLOGY OF FLOWERS. By K. Goebel, Ph.D.

Professor of Botany in the University of Munich.

There is scarcely any subject to which Darwin devoted so much time and
work as to his researches into the biology of flowers, or, in other
words, to the consideration of the question to what extent the
structural and physiological characters of flowers are correlated with
their function of producing fruits and seeds. We know from his own words
what fascination these studies possessed for him. We repeatedly find,
for example, in his letters expressions such as this:--"Nothing in my
life has ever interested me more t