THE STRUCTURE OF THE BORDERED PITS OF 
CONIFERS AND ITS BEARING UPON THE 
TENSION -HYPOTHESIS OF THE 
ASCENT OF SAP IN PLANTS 
Irvinc W. BAILEY 
(WITH PLATE I AND TWO FIGURES) 
The structure and the function of bordered pits have been 
studied by a number of investigators, particularly in connection 
with various well known theories in regard to the “‘ascent of sap”’ 
in plants. The consensus of opinion among botanists" seems to be 
that these complex pits are valvelike structures which facilitate 
the flow of water from the lumen of one tracheid or vessel into 
those of the adjoining ones. By means of the embossed areas of 
the secondary walls, a comparatively large surface of the delicate 
permeable membrane is exposed without seriously impairing the 
strength of the cell (text fig. 1). The torus or central thickened por- 
tion of the membrane acts as a valve, which may be deflected from 
its median position until it rests over one of the openings in the 
secondary walls (figs. 2, 3). In this way the delicate, permeable 
membrane is prevented from being ruptured when excessive pres- 
sure is brought to bear upon it. Furthermore, it has been generally | 
admitted by anatomists and physiologists that the membranes of | 
the bordered pits in conifers form complete septa that are impervi- ! 
ous to finely divided solids and undissolved gases. 
STRASBURGER suggested, accordingly, that the bordered pits 
functioned, in the ascent of the sap, as filters for preventing the 
penetration of air into the water-conducting elements. This view 
is also held by Drxon,? who states: ‘‘ Thus, from the point of view 
of the tension hypothesis, we regard the bordered pits as mechanisms 
to render the walls as permeable as possible to continuous water 
*See ELFvING, SCHWENDENER, PAPPENHEIM, Russow, Hartic, ScHacut, Dip- 
PEL, Santo, De Bary, StraspuRGER, Ewart, Drxon, and others. 
? Dixon, H. H., Transpiration and the ascent of sap in plants. London: Mac- 
millan. 1915 (p. 99). 
133] [Botanical Gazette, vol 62 
