30 
DR R. C. DAYIE ON 
Closed leaf-traces. 
Open leaf-traces with 
intramedullary strands. 
Open leaf-traces. 
Spiraea Aruncus, Linn. 
,, Lindleyana, Wall. 
,, sorbifolia, Linn. 
Stapliylea pinnata, Linn. 
Daucus Carota, Linn. 
Heracleum Sphondylium , Linn. 
Potentilla Anserina, Linn. 
Rosa arvensis, Huds. 
„ canina, Linn. 
„ lomentosa, Sm. 
Sambucus nigra, Linn. 
Spiraea lobata, Murr. 
We may therefore conclude that in the formation of the Dicotyledonous leaf- 
trace two factors are operative : (a) the degree of development of secondary 
thickening in the stem ; ( b ) the mode of development of the leaf (whether acropetal 
or basipetal) — cf. Col, ’04, pp. 264, 265. 
The closed type of leaf-trace appears only in woody plants, and in them only 
if the leaf is developed in acropetal fashion ; the open type of leaf-trace occurs 
especially in herbaceous plants, but also in woody plants whose leaves develop 
basipetally. A modified form of open leaf-trace is found in herbaceous plants with 
acropetally-developed leaves. 
Factors which have been found to exercise an influence .on the formation of 
the leaf- trace in Ferns and Cycads, e.g. systematic position and length of leaf and 
size of pinnae, are apparently not operative among the Dicotyledons. 
De Candolle’s tabular summary ( loc . cit., pp. 455-478) shows that within a family, 
or even within a genus, intracortical or intramedullary strands may or may not 
appear in the leaf-trace. The examples described above from the genus Spirasa show 
an analogous variability, though it must be noticed that the types of leaf-trace 
are constant within the genera into which the most recent workers have divided 
the erstwhile genus Spiraea v. 
Similarly we have seen that an elaborate type of leaf-trace may appear in a 
relatively small and simple leaf, while a simpler type is present in a large, and 
much-divided leaf. We may therefore set aside for Dicotyledons the first two of 
the four factors named * as those which operate in Ferns, Cycads, and Monocoty- 
ledons. In the Dicotyledons it is * the second pair of factors which have been 
found to influence the form of the leaf-trace. 
If we take the four factors — (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 find that the first two operate, 
sometimes jointly, sometimes independently, on the Fern leaf-trace ; the first three 
among the Cycads, (a) and ( b ) jointly in the majority of the genera, and («)„ (6), (c) 
jointly in a few cases. The fourth factor apparently alone has an influence on the 
leaf- trace in the Monocotyledons ; and the third and fourth apparently act jointly 
among the Dicotyledons. 
On p. 24. 
