422 THE ROSALES 



directions, hurling the seeds to a considerable distance (Fig. 

 317, A). This reaction is due to the tension set up by the 

 difference in the drying of the three strata of cells which com- 

 pose the walls of the pod. The Papilionaceae contain very many 

 nutritious food plants as the pea, bean and lentils, and also 

 forage plants as the clover, alfalfa and vetches. In temperate 

 regions the majority of the forms are herbaceous, only a few 

 trees and shrubs as the locust, (Robinia), broom (Cytissus), 

 dye weed (Genista), false indigo (Amorpha) belong to the 

 family, but the tropics are richly represented by a great variety 

 of woody forms that are valuable for lumber, resins and dyes, 

 as the red sandalwood, licorice root, gum tragacanth, balsam of 

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

 sensitive plant (Mimosa) and the acacias. One of the species of 

 the Acacia develops enormous spines that are tunnelled by ants 

 that protect the trees against leaf-destroying insects. Gum arabic 

 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 

 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 adheres to or 

 forms the ovary and the entire structures become succulent, form- 

 ing 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 raspberries, the ovaries are transformed into drupes which 

 may be lifted off from the convex receptacle like a thimble, while 

 in the blackberries, the drupes remain attached to the receptacle 

 which becomes the most edible portion of the fruit. The at- 

 tractive portion of the strawberry is the enlarged succulent re- 

 ceptacle while the objectionable hard particles on its surface are 

 the akenes. The receptacle surrounds the pistils in the agrimony 

 and rose, but in the former genus it is hard and covered with 



