Francis Darwin. 
35 
the conclusion that they are probably to be regarded as specialised 
branch-systems of a primitive thallus, and the departure of the 
leaf-trace is really a “ dichotomous ” branching of the vascular 
strand, or the leaves of the microphyllous Lycopods have a different 
nature, and consequently their vascular system has a distinct and 
may well have a dual origin. To suppose that the megaphyllous 
types have arisen from microphyllous types is to make an assump¬ 
tion unsupported, so far as I know, by evidence or probability. 
As regards the origin of the leaves of Filicales the hypothesis 
of primitive megaphylly appears to hold the field, and in the next 
lectures we shall see that there is some evidence for regarding 
the vascular strands of stem and leaf in the Ferns as primitively 
identical structures. 
LECTURES ON THE PHYSIOLOGY OF MOVEMENT 
IN PLANTS. 1 
By Francis Darwin. 
IV. —The Localisation of Perception. 
In this lecture I shall deal with the localisation of perception 
m geotropic and heliotropic organs. 
The earliest experiments which have a bearing on the localisa¬ 
tion of geoperception were made by Ciesielski. 2 He discovered 
two fundamental facts:— (i.) The radicles of Pisum, Lens and 
Vicia were placed horizontally, their tips having been removed, 
and were found not to curve geotropically. (ii.) Radicles were 
placed horizontally for a short time, and were decapitated before 
curvature had begun. It was found, in whatever position they 
« 
were subsequently placed, that they showed a curvature corres¬ 
ponding to the gravitational stimulus received before the tips were 
removed. The second experiment is of importance as proving that 
the loss of geotropism induced by amputation of the root-tip does 
not depend on any inability to curve in the decapitated state. 
Ciesielski’s explanation of the facts does not now concern us ; what 
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. 
2 Cohn’s Beitriige, I. 
