THE LEAF-TRACE IN SOME PINNATE LEAVES. 
25 
Descriptions of the various types of leaf-trace in the Dicotyledons have been 
given by Frank and de Candolle. The former contrasts leaf-traces which are 
open on the adaxial face with those which are closed all round. He notes the 
presence of intracortical and intramedullary strands, but does not give such an 
exhaustive account of the distribution of these as does de Candolle. The two 
authors agree in regarding the leaf- trace open on the adaxial side as more primitive 
than the closed type, and those with intracortical or intramedullary strands as more 
advanced than those which do not possess them. De Candolle gives a very extensive 
list of genera and species from many different natural orders, and shows how constantly 
a natural order preserves one type of leaf-trace. 
In comparing the leaf-trace of the Dicotyledons with those of other groups, 
one is most impressed with its plasticity in reference to the supply of the branches 
of the leaf. 
Achillea Millefolium, Linn. 
For example, in Achillea Millefolium, Linn., the leaf-trace is made up of a series 
of separate strands arranged along the abaxial side of the petiole and rachis. In the 
petiole and in the lower part of the rachis there are five large strands and some 
smaller ones. Between the pinnse the smaller strands link up with the larger and 
afterwards separate again from them. At the point where a pinna-trace is given off 
there is no sign of the smaller strands, as there they coalesce with some of the larger 
ones. The pinna-traces come from the extended margins of the strands at the ends 
of the series forming the leaf-trace, and are simply nipped off from them. 
Tripterodendron filicifolium, Radik. 
Another type of leaf-trace is found in Tripterodendron filicifolium, Radik. The 
xylem forms a closed system, roughly triangular in outline in a cross-section. 
Below a pinna the corner of this triangle nearest to the pinna becomes drawn out, 
and the narrow “V” thus formed is nipped off as the pinna-trace. A gap is left 
for a short distance above the pinna-trace, but this is soon repaired by the tissue on 
either side and the triangular outline is resumed. 
This form of pinna-supply recalls the extramarginal type of the Ferns, while the 
method in Achillea Millefolium resembles that in the marginal Fern type. 
There is, however, another common type, which is found in leaf-traces with 
intramedullary strands. 
Brownea coccinea, Jacq. 
The leaf-trace of Broivnea coccinea, Jacq., may be taken to represent this type. 
Here the leaf-trace forms a closed system, in which a small strand is enclosed. The 
main system is roughly parallel to the surface of the petiole or rachis, being flattened 
on the adaxial side and curved abaxially. The enclosed strand repeats on a smaller 
TRANS. ROY. SOC. EDIN., VOL. LII, PART I (NO. 1). 4 
