THE FRTJTT. 191 



What reason can be given for the fact that the burdock, the 

 cocklebur, the beggar-ticks, the hound's-tongue, and many other 

 common burs, are among the most persistent of weeds ? 



229. Explosive Fruits. — Some dry fruits burst open when 

 ripe in such a way as to throw their seeds violently about. 

 Interesting studies may be made of this section in the 

 common blue violet, the pansy, the wild balsam, the garden 

 balsam, the crane's-bill, the herb Robert, the witch hazel, and 

 some other common plants. The capsule of 



the South American sand-box tree bursts 

 open when thoroughly dry with a noise like 

 that of a pistol-shot. 



How are plants benefited by the explo- 

 sion of the fruit ? ' 



230. Uses of Fruits to the Plant. — 



Those portions of the fruit which surround 



the seeds serve to enclose the ovules during 



their period of ripening, and to protect 



them from drying up or from other iniuries. 

 ^ , , . , r • T 1 1 ii Fig. 178. — A Cockle- 



Other kinds of service rendered by the bur, slightly enlarged. 



coatings or appendages of the fruit may 



have been suggested by the questions asked in some of the 



preceding sections. 



Besides the dry fruits of which some of the principal kinds 

 have been mentioned, there are many kinds of stone fruits and 

 fleshy fruits, §§ 226-231. Of these the great majority are 

 eatable by man or some of the lower animals, and often- 

 times the amount of sugar and other food material which they 

 contain is very great. It is a well recognized principle of 

 botany, and of zoology as well, that plants and animals do 

 not make outlays for the benefit of other species. Evidently 

 the pulp of fruits is not to be consumed or used as food by 

 the plant itself or (in general) by its seeds. It is worth 



1 See Lubbock's Flowers, Fruits, and Leaves, Chapter III. 



