THE LEAF TRACE IN SOME PINNATE LEAVES. 
31 
In the Ferns the lack of elasticity in the vascular system and the impossibility 
in most species of expanding vascular strands once formed, have involved a strict 
economy in the distribution of the vascular tissue in the leaf and its branches. 
Every strand appears, so to speak, to be ticketed and numbered ; the supply given 
to any pinna is carefully measured in relation to the requirements of the pinna and 
in relation to the further development of the leaf as a whole. There never appears 
to be any vascular tissue to waste, but there rarely seems to be much to spare even 
for the needs of particular branches. 
The Cycad leaf-trace is, on the whole, built on the same principles as that of 
the Ferns ; it is just as provincially economical. In two genera a new method is 
adopted to meet a problem of size which is, in part at least, due to a departure 
from the method of development of the pinnae which prevails in Vascular Cryptogams 
and in some Cycads. 
The leaf-trace of the Monocotyledons is directly related to the form of vascular 
system found in the stem ; it appears to be capable of indefinite expansion and 
elaboration ; systematic position, size of leaf, sequence of appearance of pinnae alike 
are without influence upon its form. 
In Dicotyledons the vascular system of the stem seems greatly to influence the 
form of the leaf-trace, the outline of which is dictated in some measure by the degree 
to which secondary thickening is developed in the stem. But the manner of de- 
velopment of the leaf also has apparently an influence on the form of the leaf-trace, 
so that the type of leaf- trace found in any Dicotyledonous plant must be referred 
to the joint action of the two factors. The types of leaf-trace found in Dicotyledons 
are readily and easily adaptable to alterations in the size or shape of the leaf, and 
are capable of much expansion and elaboration. 
The contrast between the types of leaf-trace found on the one hand in Ferns and 
Cycads, and on the other in Angiosperms, and especially in Dicotyledons, is. very 
much one of slow and rapid growth ; it may be partly due to the size of the leaf in 
relation to the size of the whole plant. Certainly, the much greater adaptability 
of the Angiospermic vascular systems must have been no small factor in the rapid 
spread of Angiosperms over the earth. 
Summary. 
Plants of the genus Polypodium were collected in Brazil and Scotland in different 
situations ; representatives of the same species of Polypodium were found growing 
under different conditions ; the leaf-traces and pinna-traces of these species and of 
species of Aspidium, Polystichum, Dryopteris, Leptochilus, and other genera were 
examined and compared. 
The habitat of the Fern was not found in any case to influence the type of leaf- 
trace or pinna-trace. In a few species of Polypodium the habitat was found to 
have an effect on the number of tracheides in the leaf-trace. 
