KEPRODTJCTTVE ORGANS 
119 
dry, and are of considerable service 
in the discharge of spores. 
In the common ferns a heavy 
spring-like ring of cells encircles 
the delicate-walled spore case. 
When the wall becomes dry and 
comparatively brittle the spring 
straightens with considerable force, 
the delicate wall is suddenly torn 
and the spores are discharged (see 
Fig. 45). 
Even in the case of the pollen- 
spores of seed-plants, a special layer 
of the wall of the pollen-sac usually 
develops as a spring-like layer, which 
assists in opening widely the sac 
when the wall be- 
gins to yield along 
the line of break- 
ing. 
80. Discharge of 
seeds. While seeds are generally carried 
away from the parent plant by the agency 
of water currents or air currents, as al- 
ready noted, or by animals, in some in- 
stances there is a mechanical discharge 
provided for in the structure of the seed- 
case. In such plants as the witch hazel 
and violet, the walls of the seed-vessel 
press upon the contained seeds, so that 
when rupture occurs the seeds are pinched 
out, as a moist apple-seed is discharged 
by being pressed between the thumb and 
finger (see Figs. 121, 122). In the touch- 
me-not a strain is developed in the wall 
of the seed-vessel, so that at rupture it 
FIG. 124. Fruits of Spanish 
needle, showing barbed ap- 
pendages for grappling. 
The figure to the left is one 
of the fruits enlarged. 
After KERNEK. 
FIG. 125. A fruit of 
beggar ticks, 
showing the two 
barbed append- 
ages which lay 
hold of animals. 
After BEAL. 
9 
