Determining Factors in Petiolar Structure. 283 
In order to ascertain how close was this relation the total cross 
sectional area of the lumina of all the tracheae in various petioles 
was estimated and the relation which the figures so obtained bore 
to the periphery (in still air) or the area (in moving air) of the 
respective leaves was calculated. 
For this purpose transverse sections of the petioles were cut 
at corresponding levels and stained forxylem only, being afterwards 
cleared and mounted in balsam. They were then projected by 
means of a microscope and suitable optical system on to a screen 
at right angles to the axis of light so that a magnification of some 
800 diameters was obtained. 
The area of the lumen of each vessel was then traced and 
afterwards transferred so that those of each petiole together 
formed a continuous figure resembling a necklace. In this way 
the area was rendered sufficiently large to be accurately determined 
by means of a planimeter. 
Before discussing the actual figures it will be well to deal with 
a few theoretical considerations. 
Brown and Escomb have demonstrated that in still air the 
evaporation from a circular area is proportional to its diameter 
whilst if the water vapour be removed by wind the evaporation 
varies directly as the area. Either of these ratios, however, involves 
the assumption of an ideal condition for its absolute fulfilment, in 
the one case perfect stillness in the other equally perfect removal. 
In nature there is probably always more or less of a vapour 
cap covering the stomatal surface especially as the latter is most 
frequently confined to the under side of the leaf. In exposed 
situations the vapour cap is retained by means of transpiration 
checks or the rapidity of diffussion is lessened by the thickness of 
the leaf which as Renner 1 has shewn shifts the plane of saturation 
some way from the external surface. The maximum rate of transpi¬ 
ration for any leaf in moderately still air will then be broadly 
proportional to the periphery and the more rapid the movement of 
the air currents the closer will the approximation become propor¬ 
tional to the area. 
In order to eliminate as far as possible errors due to develop¬ 
ment under differential treatment the bulk of the observations have 
been made upon greenhouse plants, and by choosing those with 
opposite leaves and comparing the members of each pair, differences 
due to age are largely eliminated. 
' “ Beitrage zur Physik der Transpiration.” Flora, Bd. 100, pp. 451-548, 
1910. 
