LESSONS WITH PLANTS 349 



in dispersal. We have made use of them in another way. 

 Cottonwoods, poplars, and willows have fruit with similar 

 cotton-covered seeds. In summer we can often see the air 

 full of the floating seeds of the cottonwood, and they cover 

 the earth like snow under the trees. Where a fire has de- 

 stroyed the pines or other forests, these trees with cottony 

 seeds, poplars especially, quickly reclaim the region. 



The fruit and seeds of the catalpa are interesting. The 

 fruit is a long pod, inside which are many flat seeds with 

 each end fringed with a sail or wing. 



Pines and other conifers have a similar plan. Their seeds 

 lie on the inner face of the scales of the cones. When the 

 cone is ripe the scales spread apart and the seeds loosen from 

 the scale, taking with them a thin membranous wing for 

 flying. 



Water is another agent of seed distribution. Rains and 

 rurming streams carry the seeds of many plants. The seeds 

 of the knotweed (Polygonum) are provided with bladdery 

 wings that enable them to float. Sometimes we see the pink 

 knotweed occupying the whole length of a roadside ditch. 

 That is because a few plants originally scattered their seeds 

 upon the water which carried them along. Many other gut- 

 ter weeds are distributed in like manner. 



The seeds of willows and cottonwoods swim well and lodge 

 along the banks of rivers and the shores of lakes. That is 

 one reason why we find these trees in such locations. 



The aquatic plants, of course, depend chiefly upon water 

 currents for the scattering of their seeds. 



Animals, though generally unwittingly, are great agents 

 for the dispersal of seeds. All burs are intended to be carried 

 away by animals. These burs are the fruits of plants and 



