MIGRATION THROUGH THE AID OF WATER 185 



called samaras, are provided with wing-like appendages which often 

 contain air spaces of considerable size. These jirevent t\w frnits 

 from falling rapidly to the ground. The seeds of Cutalpa are similar]}^ 

 pro^■ided with wings. Very characteristic wind-scattered fruits are 

 found in many members of the composite family. In the dandelion 

 for example, the involucre closes after the flowers have bloomed, 

 remains closed while the fruits are maturing, and opens again when 

 the fruits are ready for dissemination. Each fruit is provided with a 

 crown of hairs, the pappus, which spreads out like a parachute and 

 prevents the fruit from falling quickly while being carried by air 

 currents. In the milkweeds (Asclepias), the willows {Salix), and the 

 poplars (Populus), the seeds are provided with tufts of hairs which 

 render them as well adapted to wind dissemination as are the akenes 

 of the dandelion and other composites. The abundant hairs on the 

 seeds of the cotton plant {Gossypium) are, from the point of view 

 of the plant, an adaptation for wind dissemination. Somewhat 

 similar cottony hairs are found on the seeds of some of the anemones 

 and of the cotton grass {Eriophoriim). 



Remarkably interesting adaptations for wund dissemination are 

 found in the so-called tumble w'eeds. These are plants which, at 

 maturity, break off from the roots and are tumbled along over the 

 ground by the wind, scattering their seeds as they go. In the case of 

 the Russian thistle {Salsola Kali tenuifolia) and the tumble weed 

 amaranth {Amaranthus groecizans) the entire shoot breaks off at 

 the surface of the soil, while in the old witch grass {Panicum capil- 

 lary) only the panicle breaks off. Sometimes the panicles of old 

 witch grass become attached to other tumble w^eeds and are carried 

 along with them. 



Many kinds of bacteria and the spores of lower plants are scattered 

 principally by the wind. Samples of air taken almost anywhere at 

 any time are found to contain greater or smaller numbers of these 

 minute organisms. 



117. Migration Through the Aid of Water. —Water is less efficient 

 than w ind as far as the numbers of propagating bodies carried are 

 concerned, but it is an important disseminating agent largely be- 

 cause it may carry fruits or seeds for long distances. In streams and 

 ocean currents the propagating bodies are all carried in a definite 

 direction but in lakes and ponds the direction often varies with the 

 changing direction of the wind. 



All seeds are heavier than air and therefore the distance to which 



