PLANT LIFE. 



Such minute seeds are readily blown long distances by 

 the wind. Relative lightness is also secured by the con- 

 struction of some seeds, which are surrounded by a volu- 

 minous coat containing many large air spaces (fig. 407). 

 Outgrowths from parts of the seed coat or pericarp also secure 

 the same end. In such cases the fall of the fruit or seed 

 though the air is so retarded that it may be carried laterally 

 some distance by the wind. No seeds, however small, float 

 long in quiet air, since buoyancy is derived only from air- 



FIG. 408. Fruits with wings. A, fruits of ailanthus tree (A. glandulosus), each carpel 

 with double wing. B, fruits of a maple tree, each carpel with a single wing. Natural 

 size. After Kerner. 



containing tissues. A flattened form of the fruit or seed is 

 very common, and this form is often exaggerated by the 

 formation of wings, i.e., of thin outgrowths from the surface 

 (fig. 408). The center of gravity in such cases is so placed 

 that the plane of flattening will be nearly horizontal when the 

 seed falls. These fruits or seeds sink from i to 30 times as 

 slowly as the same bodies without the wing. Sometimes spe- 

 cialized persistent flower leaves, either corolla or calyx, are 

 used for this purpose, as in dandelion and thistle (fig. 409). 



