DEVELOPMENT OF PLANTS 



423 



hooked bristles for distribution of the nut-like fruit (Fig. 313), 

 while in the rose it is fleshy and often brightly colored, serving 

 in some cases as food for birds and thus effecting seed distribu- 

 tion. The most pronounced changes are seen in the apple and 

 plum families. In the former group, the carpels become tough, 

 forming the "core" (Fig. 314, C) and the edible part is the 

 greatly enlarged receptacle. The plum, cherry, peach, etc., repre- 

 sent the ovary alone, the cup-like portion of the receptacle is 

 cast off as the fruit matures and the ovary becomes a drupe 

 (Fig. 314, F). In the pea family, the ovary usually becomes 

 a pod with elastic valves but it is also variously modified. In 

 the peanut, after the withering of the flower, the pistil is thrust 

 into the ground where it develops as a pod that does not open 

 at all. In other genera, the pod is nut-like as in the clovers, 

 spirally coiled in alfalfa, or separable into nut-like joints that 

 are provided with hooks as in the tick trefoil (Meibomia) (Fig. 

 317, B). Doubtless these methods of distributing their well- 



Fig. 317. Fruits of the pea family: A, fruit or pod of the ground nut, 

 showing the manner of seed dissemination by the snapping back of the 

 valves with a twisting motion. B, fruit or lomentum of tick-trefoil (Mei- 

 bomia). The lomentum breaks into as many nut-like parts as there are 

 seeds in the fruit. C, hooked bristles on the surface of lomentum. 



developed seeds, their symbiotic relations with the bacteria (page 

 57) and especially the elaborate mechanism of the irregular 

 flowers have been the causes that have led to the abundance and 



