DISTRIBUriO.V OF SFO/xES AND SEEDS. 3O3 



nation. Some (ilants, adapted to distribution by water, are 

 provided witli floats. These floats may consist eitlier of the 

 enlarged and bladdery peritarij (or some portion of it), or 

 of tlie spongy, air-fdled seed coat. The fruits or seeds are 

 thus made more buoyant and float upon the surface instead of 

 sinking as usual. Natarall)', water-loving plants are cliiefly 

 adapted to distrilauion in this manner. 



492. 3. Distribution by winds. — Some plants which secure 

 their distribution by winds are only lightly attached to the soil 

 at maturity, so that they 

 are readily uprooted and 

 carried bodily, when dry, 

 for considerable distances 

 by the wind. The transfer 

 is iacilitated by the incurv- 

 ing of the branches upon 

 drying, so that the uprooted 

 plant is more or less spheri- 

 cal in outline, or by the fact 

 that the plant is normally 

 spherical by the propor- 

 tion of the branches. Such 

 plants are known as ' ' tum- 

 ble weeds." Singly or ag- 

 gregated in large bundles 

 they are rolled o\-er plains 

 and prairies for long dis- 

 tances, shaking out their 

 seeds as they go, or open- 

 ing their fruits when moist- 

 ened. 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. 



Fig. 407.— Seeds of an orcliia (I'aiiiin teres^, 

 with cells of seed coat bladdery and filled \vith - 

 air. These seeds are ejected from thecapsule 

 by the contortions of the hairs on its inner 

 faces which cun.'e and twist as the moisture in 

 tlie air varies. Magnified 100 dlam. — After 

 Kerner. 



