DR R. C. DAYIE ON 
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scale the outline of the main, system. Below a pinna the portion of the main 
system nearest to the pinna is thrown into two longitudinal folds, the curved 
“ backs ” of which nip off, join together, and become the pinna-trace. Then the 
edge of the smaller enclosed strand nips off and passes to that part of the main 
system from which the pinna-trace has just been given off, thus assisting in the 
formation of the longitudinal folds destined to supply the pinna next above the one 
just supplied. 
The intramedullary strand thus acts as a repairing and reinforcing system, 
which helps to make good the drain on the main system caused by the departure of 
the pinnae. 
The three types of leaf- trace now described do not exhaust the types found among 
the Dicotyledons, but they are representative of the leaf-traces found in the great 
majority of Dicotyledons. 
De Candolle ( loc . cit.) has shown that the same type of leaf-trace does not invariably 
occur in all of the species of the same genus ; the examples described above show that 
the complexity of outline of the leaf- trace is not directly related to the size or degree 
of division of the segments of the leaf, for the leaf of Tripterodendron filicifolium has 
compound pinnae much larger than those of Broivnea coccinea, which are simple in 
outline. The rules which we have found to apply to Fern leaf- traces are thus 
inapplicable to the leaf-traces of the Dicotyledons. 
The leaf-traces of some species of the genus Spiraea throw an interesting light on 
the factors which control the form of the leaf-trace in the Dicotyledons. 
Spiraea Aruncus, Linn. 
The leaf-trace of Spiraea Aruncus , Linn., is composed of a large number of 
strands arranged in an ellipse (text-fig. 16 a). Below the pinnae the leaf-trace 
a b 
Text-fig. 16 . 
becomes extended laterally and from these extensions the pinna-traces are nipped 
off as replicas of the parent strand, which immediately resumes the elliptical form 
(text-fig. 166 ). 
Spiraea sorbifolia, Linn., and Spiraea Lindleyana , Wall. 
In Spiraea sorbifolia, Linn., and S. Lindleyana, Wall., the leaf- trace is more or 
less triangular in outline in cross-section. The pinna-traces leave the corners nearest 
to the pinnae, while the leaf-trace quickly re-forms on their departure. 
