2 14 
James Small. 
mentioned previously (Chap. V, A) the pappus is sensitive to a 
difference of -03 in the R.H., opening spontaneously at R.H. -75 
and closing at R.H. "78 when opened by artificial drying. The third 
speed of the fan was used. 
Critical Pressure A—*1318 gms. per sq. cm. 7-2 m.p.h. 
Critical Pressure B—-0232 „ „ 3‘0 ,, 
Ursinia speciosa, D.C.—This was taken as an example of a 
fruit with a distinctly paleaceous pappus. The fruit has a small 
tuft of hairs at the base and five, well developed, white and gold 
plumoso-paleaceous scales at the top. The fruit is not particularly 
heavy, but the pappus is relatively solid, and its weight is somewhat 
of the same magnitude as that of the fruit itself, not, as in the 
other cases, very much smaller than that of the whole structure. 
The spreading of the pappus seems to be more or less permanent 
in this case, and no pulvinus could be detected. The third speed 
of the fan was used. 
Critical Pressure A—*0514 gms. per sq. cm. 4-5 m.p.h. 
Critical Pressure B—-0118 ,, „ 2-1 „ 
Leontopodium alpinum , Cass.—The fruit in this case is small 
and more or less spherical with no pappus. The average diameter 
of the fruit is about ’25 mm. The small size of the fruit prevented 
accurate measurements with the apparatus as arranged. With the 
tube at 33*5 cms. and the fan at Speed 4 six fruits were dropped 
through the hole. This was repeated three times with the result 
that about half the fruits were blown right through the tube, and 
the others were blown a distance which varied from 35 cms. to 90 
cms. The critical pressure A is, therefore, about -050 gms. per sq. 
cm., equivalent to 4'4 m.p.h. Six fruits were dropped through 
the hole with the tube at 33-5 cms. and the fan at Speed 1. Three 
of them were blown about 5 cms. and the others were lost. The 
\. 
critical pressure B is, therefore, not more than -0168 gms. per. sq. 
cm., equivalent to not more than 2 59 m.p.h. 
C. Hydrodynamics of Wind-Dispersal. 
According to the views of Dingier and Praeger the dandelion 
fruit, which falls in quiet air at the rate of "98 m.p.h., would require 
an uniform wind of about 50 m.p.h. and an initial elevation of 
about one mile in order to be transported about 50 miles, or 
1 cm. elevation for 50 cms. dispersal. In other words the ratio 
distance travelled 
initial elevation 
is a measure of the velocity of the wind required. 
