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 

 -While seeds 



Pio. 125. A fruit of 

 b e g g :i r ticks, 

 showing the two 



barbed append- 

 ages which lay 

 hold of animals. 

 —After Beal. 

 Q 



Fig. 124. Fruits of Spanish 

 needle, showing barbed ap- 

 pendages for grappling. 

 The figure to the left is one 

 of the fruits enlarged. — 

 After Kerner. 



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 

 linger (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 



