32 
DR R. C. DAYIE ON 
The type of pinna-trace is constant within a genus, recognised as such in 
Christensen’s Index Filicum ; the form of the leaf- trace is dependent on the length 
of the leaf and the size of the pinnae. 
The form of the adaxial portion of the leaf-trace is constant throughout a genus ; 
the degree of development of the abaxial curve depends on the length of the leaf 
and the size of the pinnae. 
The leaf-traces of species of Stangeria , Cycas, Zamia, Ceratozamia, Macro- 
zamia, Dioon, Encephalartos , and Bowenia have been compared with those of the 
Ferns. In the Cycads the abaxial portion of the leaf-trace is directly connected with 
the length of the leaf ; the pinna-trace is, except in Encephalartos and Bowenia , 
supplied from the adaxial margin of the leaf-trace nearest to the pinna ; in Encephal- 
artos and Bowenia a reinforcing systeip. is developed on the adaxial face of the leaf- 
trace. The adaxial reinforcing system is related partly to the size of the pinnaB and 
partly, in Encephalartos , to the mode of development of the leaf. 
Species of Cocos, Phoenix, Caryota, and Philodendron have been examined ; in 
them the forms of leaf-trace and pinna-trace are directly related to the type of 
vascular system found in the stem, and appear to be independent of the size of leaf 
and the manner of its development. 
Two distinct types of leaf-trace, with their varieties, have been found in Dicoty- 
ledons, and have been studied especially in species of the genera Achillea, Triptero- 
dendron, Brownea, Spiraea, Astilbe, Rosa, Daucus, and Heracleum. 
The type of leaf-trace with strands forming a ring is found in woody plants, 
excepting those with basipetally-developed leaves ; the type open on the adaxial 
face is found in herbaceous and woody plants with basipetally-developed leaves ; in- 
herbaceous plants with acropetally-developed leaves the “open” type of leaf- trace 
is provided with an intramedullary reinforcing system. 
The factors which control the form of leaf-trace and its system of branching are : — 
(a) Systematic position. 
( b ) The length of the leaf and the size of its appendages. 
(c) The order of development of the pinnae. 
(d) The type of vascular system found in the stem. 
(a) and (b) are operative among Ferns ; (a), (b), and(c) among Cycads ; (d) among 
Monocotyledons ; and ( c ) and ( d ) among Dicotyledons. 
