THE SEED 25 
voluntary agency of man. Dispersal by this means, whether 
intentional or accidental, is purely artificial, and except in the 
case of a few annuals like horseweed, bitterweed, ragweed, 
goosefoot, and other field pests that have adjusted their sea- 
son of growth and flowering to the conditions of cultivation, 
is not correlated with any special modification of the plants 
for self-propagation. On the contrary, many of the most 
widely distributed weeds of cultivation, such as the ox-eye 
daisy, the rib grass, mayweed and bitterweed, possess very 
imperfect natural means of dispersal, and are largely depend- 
ent for their propagation on the involuntary agency of man. 
23. Use of the fruit in dispersal. — It will be seen from the 
foregoing observations that the fruit plays a very important 
part in the work 
of dispersal, most 
of the adapta- 
tions for this pur- 
pose being con- 
nected with it. 
In cases where a 
number of seeds 
are contained 
in a large pod “= 
that could not UTED TAR 
conveniently be aa oe 
blown about by showing the bunchy top and Fic. 44.— Panicle ot 
weak anchorage of a typical “ old witch grass,”’ a com- 
the breez ©, tumbleweed. mon tumbleweed. 
adaptations for 
wind dispersal are attached to the individual seeds, as in the 
willow, milkweed, trumpet creeper, and paulonia; but as a 
general thing, adaptations of the seed are for protection, the 
work of dispersal being provided for by the fruit. In the case 
of the large class of plants known as “ tumbleweeds, ” the 
whole plant body is fitted to assist in the work of transporta~- 
tion. Such plants generally grow in light soils and either 
have very light root systems, or are easily broken from their 
