James Small. 
224 
elongation takes place.” Cassini (8) had previously recorded the 
phenomenon in Tussilago Farfara and Chevreulia stolonifera but 
gave no details. 
The Hydrodynamics of Fruit-Dispersal in other Species. 
The theory of the experiments with the other fruits is com¬ 
plicated by the angle at which the pappus spreads in Senecio, 
Ursinia and Centaurea, by the presence of mucilaginous achenial 
hairs in Senecio, the presence of a basal tuft of hairs in Ursinia 
and the absence of a pappus in Leontopodinm. The structure is 
in no case that of a simple parachute with the wind-holding 
surface at right angles to the axis of the fruit, as in Taraxacum. 
Senecio vulgaris, L.—According to Praeger the fruit of Senecio 
vulgaris falls in quiet air at the rate of 12 feet in 12 8 seconds, 
which is approximately '66 m.p.h. This is considerably lower than 
the value (1*18 m.p.h.) obtained for the velocity equivalent to the 
critical pressure B in this species. There are several sources of 
inaccuracy, but if '66 m.p.h. is taken as the velocity of the 
critical vertical component, it is possible to make a rough 
calculation of the minimum wind for dispersal. The efficiency of 
the pappus can be taken as approximately '25 and calculating as 
in the case of Taraxacum, 1 the minimum wind for dispersal is 1-25 
m.p.h. This is quite a good approximation to the value (1.48 
m.p.h.) found for the velocity equivalent to the critical pressure A 
in the experiments described above. 
Centaurea imperialis, Hausskn.—As Praeger does not give the 
rate of fall in this species an approximate measurement of this 
constant was made by his method and the value found was 2-2 
seconds for a fall of 12 feet. This is equivalent to 3*81 m.p.h. 
Calculating as before we get 7-3 m.p.h. as the minimum wind 
for dispersal. These two values (7'3 and 3*81) are sufficiently 
near the experimental values (7-2 and 3*0) found for the 
velocites equivalent to the critical pressures A and B to show that 
the methods of experiment and calculation are both approximations 
to the actual values of the two constants. 
Ursinia speciosa, DC. —The rate of fall was determined in this 
case also for the purpose of this calculation. The value found 
was 3-4 seconds for a fall of 12 feet. This is equivalent to 2-47 
m.p.h. Calculating as before we get 4‘7 m.p.h. as the minimum 
wind for dispersal. These two values (4’7 and 2-47) are quite good 
’ the tilting of the fruit need not occur in this case as the pappus is 
already at an angle with the horizontal. 
