COLONIZING SPECIES. 573 
most obvious one to the casual observer is the pappus, enabling 
the seed* to be carried off by winds. But A. De Candolle, in his 
above-mentioned * Géographie Botanique, had already shown how 
inefficient this is for transport across seas; and his views have 
been fully confirmed by the observations of Kerner. The most 
violent winds will not earry them above two or three miles; the 
moment the pappus gets into a damper atmosphere it collapses, 
and when once the seed has fallen to the ground it is very rarely 
again raised by the wind. The most that the pappus does to 
assist the species in its migration is to carry the seed to a short 
distance from its parent and deposit it in running water, on bales 
of goods, on soil to be taken as ballast, or in other places where it 
has a chance of being carried further by other means. That the 
pappus, indeed, is really and solely a provision for the transport 
of the seed will searcely be maintained when we consider, first, 
that in the great majority of more or less unisexual Composite the 
pappus is much more developed on the male or sterile achenes than 
on the female fertile ones, and that in alarge number of Cynaroi- 
dex, and even in many Cichoriacez, the pappus separates so readily 
from the achene that the down we see floating in such quantities 
over a field of thistles has, for the most part, left the achene 
enclosing the seed behind. So little, indeed, does the pappus 
really assist the emigration of species, that we find in the above 
list of sixty colonists only twenty-two or twenty-three, or but little 
above one third, possessing an available pappus ; whilst in the total 
number of known Composite more than two thirds are so endowed. 
Adherence to moving or moved objects by means of hooked 
points, spines, glutinous exudations, intricate scabrous-fibred 
wool, &c. is a well-known cause of the transport of Composite 
fruiting heads, and has so much assisted, for instance, the spread 
of Xanthium and other nuisances; but even that alone is not so 
frequent as is supposed. Some half a dozen only of the above list 
have adherent heads, a few more (Bidens, Calendula, Adenostemma) 
have adherent achenes; and adherence alone is quite insufficient. 
Many Composite burrs, such as Acanthospermum, have but a com- 
paratively limited area. As in Boraginee, the burr-nutted 
Echinospermums are, as a rule, much less generally diffused than 
the smooth-nntted Myosotises; so, in Composite, Arctium lappa 
migrates much less than Cnicus arvensis; Calotises have a more 
limited area than many Brachycomes. 
* I use the word “seed” here in the popular sense, which, in Composite, is 
the botanical achene, including the inseparable pericarp. 
