24 
DR R. C. DAVIE ON 
side of the leaf-trace is constant within a genus, but the reinforcement supplied from 
the abaxial curve varies in accordance with variations in individual leaves. 
In the Cycads the general outline of the leaf-trace is constant within a genus. 
The degree of development of the ^baxial curve depends upon the length of the leaf 
and the size of the pinnae. The method of forming the pinna-trace is constant in all 
the genera examined except Encephalartos and Boivenico, where the size of the 
pinnae causes the development of an adaxial reinforcing system. There is some likeli- 
hood that in some leaves the presence of the reinforcing system is related to a 
basipetal sequence in the development of the pinnae. 
In the Monocotyledons the form of the leaf-trace is directly connected with the 
scattered bundle system characteristic of the group, and is independent of the size of 
the leaf or of the order of development of its segments. 
The factors, therefore, which control the form of the leaf-trace in the groups 
which have been examined are (a) systematic position, (b) the length of the leaf and 
the size of its appendages, (c) the order of development of the pinnae, and ( d ) the 
type of vascular system found in the stem: 
We may now proceed to discover which of these factors are operative among the 
Dicotyledons, in which the forms of leaf-trace are more varied than in any other 
group of plants. 
The Leaf-Trace in the Dicotyledons. 
The earliest comparative papers are by Frank (’64), de Lanessan (’74), and de 
Candolle (’79). In 1888 Lignier pointed out the relationship between the vascular 
system of the stem, the type of leaf-trace, and the arrangement of the leaves on the 
stem. The position of leaf- trace structure among the criteria of value in systematic 
analysis was discussed by Vesque in 1882. From that period forwards there is 
a continuous increase in the number of contributions to our knowledge of the leaf- 
trace in different families of Dicotyledons. These have been made by van Tieghem 
(’84 and ’93) — to whose inspiration most of this work is due, — Dumont (’87), Petit 
(’87), Thouvenin (’90), Parmentier (’96 and ’97), Perrot (’98), Decrock (’01), 
Gaucher (’02), Jodin (’03), Viguier (’06 and ’09), Pellegrin (’08), Guillaumin (’09), 
and Le Renard (’13). General questions have been discussed by Bonnier (’00), who 
dealt with the relation of form to function in the leaf-trace and its branches ; 
Bouygues (’02), who described the distribution of meristems in petioles, showing the 
type of leaf- trace with strands arranged in a circle to be an advance on the type open 
adaxially ; Chauveaud (’ll), who contrasted the evolution of the Fern leaf- trace with 
that of the Phanerogams ; and Parmentier (’96) and Sarton (’05), who estimated 
the value of anatomy in the delimitation of critical forms. An admirable and richly 
illustrated general account of petiolar anatomy has been given by Col (’04), who 
referred the variations in complexity of Dicotyledonous leaf-traces to slow or rapid 
growth of the leaves. 
