278 GENERAL BIOLOGY 



of changes of moisture, growing most luxuriantly 

 in the greatest degree of warmth and moisture as 

 in the humid climate of the tropics. In the abun- 

 dant vegetation of these regions there has arisen 

 a sharp " struggle for light " which has brought into 

 being a great variety of climbing-plants and vines, 

 and of aerial plants that depend for water upon 

 what their hair-covered aerial rootlets draw from the 

 moist atmosphere. 



Adaptations for Seed Dispersal. Unable to move 

 from the place in which they are rooted, plants are 

 nevertheless able to spread their various species 

 over available territory with great rapidity, chiefly 

 on account of special adaptations in connection with 

 the dispersal of their seeds. 



Many seeds are formed to float in the wind and 

 are transported long distances in this way. A 

 familiar example is " thistle down " and the cottony 

 seed of the milkweed. Often the seed is winged, 

 as in the case of the maple or the catalpa. In some 

 cases, the whole plant is transported by the winds. 

 On the western plains, the Russian thistle or '* tum- 

 ble-weed " (there are several sorts) grows into a 

 large globular bush which breaks off close to the 

 ground when ripe and goes rolling across the prairie, 

 scattering its seeds as it goes, until brought to a 

 standstill by a fence. 



Other seeds are wonderfully adapted to stick to 

 anything that touches them, particularly the furry 

 coat of animals. The " beggar's lice " (Desmodium] 



