DISTRIBUTION OF SPORES AND SEEDS. 363 



nation. Some plants, adapted to distribution by water, are 

 provided with floats. These floats may consist either of the 

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

 of the spongy, air-filled seed coat. The fruits or seeds are 

 thus made more buoyant and float upon the surface instead of 

 sinking as usual. Naturally, water-loving plants are chiefly 

 adapted to distribution 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 facilitated 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 over plains 

 and prairies for long dis- 

 tances, shaking out their 

 seeds as they go, or open- , 5tfK£*te{^2tf»53a 

 ing their fruitswhen moist- ^£Eg2r£SflSZS& 



~r»^ A nntlipr nrhnrstinn f aces which curve and twist as the moisture in 



enea. Anotner adaptation the air varies Magnified IOO diam.— After 

 for distribution by the Kerner - 



wind is the small size of some seeds. Those of some orchids 

 are so diminutive that it takes 500,000 to weigh 1 gram. 



