THE LEAF-TRACE IN SOME PINNATE LEAVES. 
5 
has already been described for various Angiospermic plants. Vesqtje and Viet (’ 81 ) 
have shown that the vascular strands of Pea plants grown in a humid atmosphere are 
narrower than those of plants grown in a dry atmosphere. Similar results were 
obtained by Kohl (’ 86 ) and Eberhardt (’ 03 ), while more recently Gain (’ 95 ) and 
Cannon (’ 05 ) have made experiments on the influence of water in the soil upon the 
structure of some stems. Cannon found that “ non-irrigated ” desert plants have 
larger ducts and more of them per equivalent area of cross-section than “irrigated” 
plants. While this record agrees with the account of the forest and restinga Ferns, 
detailed above, no close comparisons are really permissible between the observations, 
because Cannon worked with the stems of plants which had an annual increment of 
growth. His explanation of the differences between the plants with which he made his 
experiments is notin any case applicable to the Ferns of forest and restinga, since he 
assigns the contrast between the irrigated and non-irrigated plants to differing rates of 
growth, the curve of growth for the non-irrigated plants being very similar to that of 
the yearly rainfall, while the irrigated plants grow evenly throughout the year. In the 
case of the Brazilian Ferns it certainly appears as if different rates of transpiration 
were responsible for the contrast between the forest and restinga types. No matter 
what may be the water-supply to the leaves, Ferns growing on the restinga or on 
the loose sand of the Brazilian beaches' should lose much more water than those 
which grow in the moist depths of the forest. Rapid transpiration would involve the 
provision of a large amount of xylem in a leaf-trace, while the risk of desiccation on 
the hot wind-swept restinga might cause the employment of tracheides as water- 
storing elements, in place of thin-walled cells. This simple explanation would 
certainly fit the cases of the Ferns described above. Water-storage in tracheides is 
knowm in various groups of plants (see Kruger, ’ 83 ; Kny and Zimmermann, ’ 85 ; 
Vesque, ’86 ; Volkens, ’86 ; Gilg, ’91 ; Warming, ’93 ; Rothert, ’ 99 ), and might quite 
well occur in the leaves of these exposed Brazilian Ferns. The relative amounts of 
xylem in their leaf-traces form an interesting parallel to Cannon’s observations, 
which, however, have been subjected to criticism by Groom (TO). In the absence 
of observations on the rates of transpiration in the two series of leaves and on the 
water content of the soil on -the restingas and in the forest, we can merely record the 
results of the examination of these Fern-leaves. For the purpose of the present 
paper it is sufficient to note that the very different conditions, w^hile they do markedly 
influence the structure of the leaf-trace, have no effect on the type of pinna-trace. 
In these leaves, as in the leaves of all the species of Polypodium which have been 
examined, the margins of the leaf-trace go off to supply the pinnae. The marginal 
type of pinna-trace is found uniformly throughout the genus. 
The leaf-traces of the ten species of Polypodium collected in Brazil are represented 
diagrammatically in text-fig. 1. Text-fig. la represents the leaf-trace of P. serru- 
latum, which has the smallest leaf of all the series and which has very tiny “ pinnae ” 
— mere teeth projecting slightly from the sides of the rachis. In the leaf- trace 
