Origin and Development of the Compositev. 223 
i.e., the distance travelled by the fruit is equal to the initial eleva¬ 
tion. The force X is different from the pressure M or W in that 
the fruit is being carried along as a balloon is carried along with a 
velocity equal to the velocity of the wind. 
From the above it is clear that what is really measured in 
the experiments on the critical pressure B described in Section B is 
not the minimum vertical component of M but the rate of fall, 
which is, however, equal to the latter constant. It would be possible 
to measure the critical vertical component directly with a modified 
apparatus, and the writer hopes to continue the investigation. 
The conclusion to be drawn from the experimental and 
theoretical treatment of the hydrodynamics of the dispersal of 
the dandelion fruit may be stated briefly thus—so long as the 
relative humidity of the air remains above *77 and so long as the 
fruit does not encounter an obstacle, a horizontal wind of F97 
m.p.h. is sufficient for its dispersal to any distance. 
If the air becomes moist the pappus closes up and the fruit 
falls rapidly. The problem of surmounting low obstacles is partly 
solved by the elongation of the scape, which takes place during the 
twenty-four hours before the fruits are ready for dispersal. The 
scape elongates 50% to 90% of its original length, e.g., from 24 cms. 
to 38 cms. and from 20 cms. to 38 cms. in cases which were 
measured. This sudden elongation is due to a stretching of only 
the upper part of the scape : the stretching causes the elongated 
part to be paler incolour since the chlorophyll is not increased in 
proportion to the increase in surface. The physiology of the 
phenomenon seems to be somewhat as follows—during the 
ripening of the fruits there is a marked streaming of carbohydrates 
up the scape to the capitulum. This streaming does not cease 
immediately on the maturation of the fruits, and the materials 
accumulate in the upper part of the scape. This causes the osmotic 
pressure and the turgidity of the cells in that region to increase so 
much that the cell walls are stretched along the line of least 
resistance, i.e., longitudinally. Accurate experiments are, however, 
necessary to prove this conclusively, and the point appears to be 
of general interest in view of the similar quick elongation of the 
supporting stalk in the spore-dispersal of many Bryophyta, and 
the similar but slow elongation in the peduncles during the 
fruiting stage of many Angiosperms. Beeby (3) observed similar 
elongation in the scape of T. spectabile, var maculiferum, but was 
“ not able to give exact details concerning the times at which 
