A. J. Grove 483 
“When the stylets have been forced into the desired part of the plant- 
tissues, suction commences. 
Owing to the very minute lumen composing the suction-canal, it is very 
probable that the plant juices ascend up this canal, largely by means of 
capillarity, and there is the further factor of the pressure of the sap itself. 
As the divaricator muscles of the pharynx contract, they pull out the 
flexible dorsal wall of the pharynx, thus greatly enlarging the lumen of that 
structure. This causes a sucking action, and as the plant-juices make their 
way from the suction-canal up the pharyngeal duct, they are drawn from the 
latter into the pharynx proper.” 
It is clear that in considering the question of the mechanism of suction, 
two problems present themselves. First, the flow of the sap from the host 
along the suction canal, and, second, the ejection of the saliva down the 
salivary canal. In connection with the first, it is seen that Awati and Davidson 
both suggest that capillarity and the pressure of the sap are important factors, 
and this would certainly appear to be the case. The extent to which capil¬ 
larity assists in the process is difficult to determine, since there are so many 
factors, some of which would vary under different circumstances, about which 
accurate information is not easily obtainable. It is known that the height to 
which a liquid will ascend a fine tube by the action of capillarity is dependent 
upon the surface tension and the density of the liquid, the diameter of the 
tube, and the angle which the liquid makes with the tube. Of these, only the 
diameter of the tube can be readily determined. The others: surface tension, 
density, and angle of junction with the tube would vary according to the 
composition of the sap. Since, however, the rise of the liquid in the tube is 
inversely proportional to the diameter of the tube, it is clear that in the case 
of a tube of such a fine bore as that of the suction-canal, the distance to which 
the liquid will rise will be appreciable, unless the liquid is very viscous. The 
part played by the pressure of the sap will also be a variable quantity, de¬ 
pending upon the kind of tissue to which the setae penetrate. If they enter 
the sap-bearing xylem, the pressure of the sap might be a definite factor, but 
if they merely puncture the cortical cells, and lacerate them, it is not at all 
easy to see how the osmotic pressure of the cells could be of any assistance, 
for once the cells were torn, the cell-sap would pour out, but the osmotic 
pressure would disappear. In neither of the descriptions quoted above, is the 
suctorial effect produced by the pharynx suggested as assisting in the con¬ 
veying of the sap along the suction-canal, but it is possible that it may do so. 
The chief difficulties in the way are that there is no physical continuity be¬ 
tween the pharynx and the suction-canal, and that the suction-canal itself is 
—at least in P. mali —not a complete tube throughout its whole length. Under 
these circumstances, it would seem that any suctorial effect produced by the 
pharynx would not be transmitted to the fluid in the suction-canal, but would 
merely draw in such fluid, or air, that surrounded the orifice of the pharynx 
at the time. But if the conditions which obtain during the process of suction 
