280 



OUTLINES OF PLANT LIFE. 



mally spherical by the proportion of the branches. Such 

 plants are known as ' ' tumble weeds. ' ' Singly or aggregated 

 in large bundles they are rolled over plains and prairies for 



long distances, shaking out 

 their seeds as they go, or 

 opening their fruits when 

 moistened. 



Another adaptation for 

 distribution by the wind 

 is the small size of some 

 seeds. Those of some 

 orchids are so diminutive 

 that it takes 500,000 to 

 weigh I gram. Such 

 minute seeds are readily 

 blown long distances by 

 the wind. Relative light- 

 ness is also secured by the 

 construction of some seeds, 

 which are surrounded by a 

 voluminous coat contain- 

 ing many large air spaces 



Fig. 242,— Seeds of an orchid \Vanda teres"), icl _ .„\ r>iitmTiwt>ic 



with cells of seed coat bladdery and filled with (."g- 242J. UUtgrOWmS 



air. These seeds are ejected from the capsule fmrn narts nf tVi<= spprl mat 



by the contortions of the hairs on its inner 1^0™ parts 01 tne SeeO COai 



faces which curve and twist as the moisture in r^a-r\r-3-r-n alor. Cf>/-iirf 



the air varies. Magnified 100 diam.— After Oi pericarp aiSO secure 



'^"°"- the same end. In such 



cases the fall of the fruit or seed through 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 buoy- 

 ancy is derived only from air-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 out- 

 growths from the surface (fig. 243). The center of gravity 

 in such cases is so placed that the plane of flattening will be 



