22 I 
Origin and Development of the Composites. 
Composite, another example of the efficiency in detail which is 
characteristic of the family. 
With 0=45°, V=M cos# sin0=*5 M 
H=M (cos#) 2 =*5 M 
H=V and M—2 V 
the value of V as determined experimentally is *0026 j<ms, per 
sq. cm. 
.*. H= # 0026 gms. per sq. cm. and M=*0052 gms. per sq. cm. 
The efficiency of the pappus as calculated from the known 
value of V for the area of the pappus and from the weight of the 
fruit is -27. 
.*. H'='0026 x •27= , 0007 gms. per sq. cm. 
The pressure M depends on the velocity with which the wind 
overtakes the fruit as it moves along and the velocity of the effective 
part of the wind is the difference between the velocity of the wind 
and the velocity of the fruit, i.e. m=w—h' (.*. w=m -|-h' ). 
The pressure M (-0052 gms. per sq. cm.) is equivalent to a 
velocity of 1-44 m.p.h., i.e., m=l , 44 m.p.h.; the pressure H' 
(•0007 gms. per sq. cm.) is equivalent to a velocity of ’53 m.p.h., 
i.e., h'=-53 m.p.h., but w—m + h, .\w=T44 + *53=l , 97 m.p.h. 
In this way we find that the minimum wind for dispersal has 
a velocity of 1-97 m.p.h.,which is equivalent to a pressure of ‘0097 gms 
per sq. cm. Referring to Table X we find the critical pressure A 
is .0107 gms. per sq. cm., which is equivalent to 2 06 m.p.h. At 
the next point taken in the experiment the velocity had fallen to T84 
m.p.h.: the fruit skimmed along the tube, touching it with the base 
only, being thus partially supported and leaving the wind free to exert 
a pressure sufficient to blow the fruit right out of the tube. The 
critical pressure A in the case of Taraxacum officinale is clearly an 
approximation to the pressure of the minimum wind required for 
dispersal. Whether it is so for other species remains to be 
determined. 
In the case of the dandelion we thus arrive at the interesting 
conclusion that the critical pressure A is approximately equal to W, 
the pressure exerted by the minimum wind necessary for dispersal, 
and that the critical pressure B is approximately equal to V, the 
pressure of the minimum vertical wind which will support the fruit 
i.e., the pressure of the minimum or critical vertical component of 
the effective part M of the horizontal wind W. In the language of 
aeronautics 1-44 m.p.h. is the minimum speed of the dandelion 
fruit; the difference in the two cases is that so long as the aeroplane 
