DISTRIBUTION OF SPOKES AND SEEDS. 



363 



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

 provided with floats. These floats may eonsist 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 arc rolled over 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. Thoseofsome orchids 

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



Fig. 107.— Seeds ol an 1 ■'■ > **\ 



with cells ol seed coal bladdery and filled with 

 air. These seeds are eje< ted From the < apsule 

 by the contortions ol the hairs on its inner 

 faces which curve and twist as the moisture in 

 the ah varii s. Magnified diam Ifter 



