DEVELOPMENT OF PLANTS 



455 



indigo, etc. Related to the pea family are the curious vSensitive 

 plant {Mimosa) and the acacias. One of the species of the 

 Acacia develops enormous spines that are tunnelled by ants 

 that were supposed 'to protect the trees against leaf-destn)>ing 

 insects. Gum arable is obtained from African and Australian 

 species of Acacia. 



One of the most interesting features of the Rosales is the. 

 variety of changes to which the receptacle and ovaries are subject 

 in the ripening of the fruit. In the majority of cases, the pistil 

 ripens as an akene or follicle (splitting along one side to free 



Fig. 317, Fruits of the pea family: .1, fruit or pod of the ground nut, 

 showing the manner of seed dissemination by the snapping back of the vah'es 

 with a twisting motion. B, fruit or lomentum of tick-trefoil {Meihomia) , 

 The lomentum breaks into as many nut-like parts as there are seeds in the 

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



the seeds) or as a pod, without any considerable modification in 

 the form of the ovary. In many instances, however, this growth 

 is attended with pronounced alterations of the parts as in the 

 currants and gooseberries where the receptacle forms the ovary 

 and the entire structure becomes succulent, forming a berry. 

 The same organs in the witch-hazel develop into a horn-like 

 capsule, the walls of which, after splitting open, contract and 

 pinch out the hard, smooth seed with great force. In the rasp- 

 berries, the ovaries are transformed into drupes which may be 

 lifted off from the convex receptacle like a thimlile, while in 



