215 
Origin and Development of the Compositce. 
In the apparatus described above the initial elevation is at 
most 4-5 cms. (the diameter of the tube) and the maximum distance 
travelled can be taken as the length of the tube, 125 cms. The 
wind-velocity necessary for the transport of the dandelion fruit to 
125 
the end of the tube would then be ^=28 m.p.h. approx. The 
experimental result, as shown in Section B, is 2'06 m.p.h. approx. 
This discrepancy requires some explanation and the essential point 
lies in the fact that the wind-dispersal of pappose fruits has been 
regarded hitherto as a hydrostatic problem and not as the 
hydrodynamic problem which it undoubtedly is. The average 
pappose fruit is more akin to an aeroplane or a kite than to a 
parachute or balloon. An aeroplane has a much greater rate of fall 
in quiet air than a dandelion fruit but a wind of 60 m.p.h. is 
sufficient to keep the former in the air indefinitely. It must also be 
noted that as long as the “air speed” is 60 m.p.h. aq aeroplane 
remains up, so that in a wind of 90 m.p.h. an aeroplane could 
actually drift backwards at the rate of 30 m.p.h. in relation to 
the earth. This case is somewhat similar to that of the pappose 
fruit (see below); there would be a considerable element of danger 
in the above-mentioned stunt but I am assured by aeronauts that 
it is possible. 
Hydrodynamics of Fruit-Dispersal in 
Tauaxacum officinale. 
The dandelion fruit is taken as the simplest example. Here 
the main weight of the fruit is at the base of the slender stalk, 
while at the top of the stalk there are the numerous hairs of 
the pappus which spread out when the R.H. of the air is 'll or 
less. These hairs form a flat, circular surface at right angles to 
the main axis of the fruit; the centre of gravity is low, so that the 
structure is that of a simple parachute and is relatively stable. 
Now if the fruit is considered to be vertical at first (Fig. 27, A) 
and a wind with a velocity which develops a pressure M impinges 
on the fruit in such a position, the pappus being lighter than the 
heavy fruit body, the fruit does not remain erect. The pappus is 
acted upon more strongly and is blown ahead of the fruit body, so 
that the whole fruit becomes tilted as in Fig. 27, B. Once this 
position is reached the force M can be resolved into two components 
S and R, S being parallel with the surface of the pappus and 
R at right angles to that surface. The two components are 
equal to M sin# and M cos# respectively,# being the angle of the 
