SEEDS SCATTERED BY EXPLOSIVE MECHANISMS 87 
collect on the wagon wheels and are carried to the highways or 
to other fields. Threshing machines are important agents in 
scattering seeds, for in their traveling through the country seeds 
of various kinds are jostled 
from them and seed the 
fields and highways. 
Man scatters many 
weeds by sowing unclean 
seed. Clover seed, Alfalfa 
seed, Grass seed, Wheat, 
Oats, etc., are often ob- 
tained from distant states 
or even from foreign coun- 
tries for seeding. Weed 
seeds are usually present Fic. 89.— The threc-valved pod of the 
in agricultural seeds, and Violet throwing its seeds. Much enlarged. 
: After Beal. 
sometimes they are pres- 
ent in large quantities. In tracing weeds, it has been found 
that many of the most troublesome ones have come from Europe, 
Asia, or some other foreign country. Man has carried the seeds 
and fruits of these weeds across the seas, and most of them have 
been imported and sown with agri- 
cultural seeds. 
Seeds Scattered by Explosive 
or Spring-like Mechanisms. — In 
this kind of dissemination the 
plant itself is the agent which, 
either by sudden ruptures due to 
strains or by explosions due to 
the swelling of certain tissues, is 
able to throw the seeds often a 
considerable distance. In the 
pods of some plants, as in the 
Vetches, Witch-hazel, Castor 
Fic. 90.—The Squirting Cu- Bean, and Field Sorrel, bands of 
eumber (Ecbalium Elaterium) tissue, which ripen under tension, 
papeiniiniey Ue Sous atom eter (oe exert such a strain that the pods 
suddenly rupture with so much violence that the seeds are thrown 
in every direction. In the Violets the carpels, as they ripen and 
dry, press harder and harder upon the seeds, which suddenly 
