REPRODUCTIVE 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 



FIG. 125. A fruit of out, as a moist apple-seed is discharged 

 beggar ticks, ^ being pressed between the thumb and 



showing the two J . 



barbed append- finger (see Figs. 121, 122). In the touch- 

 ages which lay me . not a strain is developed in the wall 



hold of animals. * 



After BEAL. of the seed-vessel, so that at rupture it 

 9 



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. 



