370 



ECOLOGY. 



up as they are whirled along, often miles away. In late spring or early 

 summer the pods of the willow burst open, exposing the seeds, each with a 

 tuft of white hairs making a mass of soft down. As the delicate hairs dry, 



Fig. 472. 



Touch-me-not (Impatiens fulva) ; side and front view of flower below ; above unopened 

 pod, and opening to scatter the seed. 



they straighten out in a loose spreading tuft, which frees the individual seeds 

 from the compact mass. Here they are caught by currents of air and float 

 off singly or in small clouds. 



676. The prickly lettuce. In late summer or early autumn the seeds of 

 the prickly lettuce (Lactuca scariola) are caught up from the roadsides by 

 the winds, and carried to fields where they are unbidden as well as unwel- 

 come guests. This plant is shown in fig. 473. 



677. The wild lettuce. A related species, the wild lettuce (Lactuca cana- 

 densis) occurs on roadsides and in the borders of fields, and is about one 

 meter in height. The heads of small yellow or purple flowers are arranged 

 in a loose or branching panicle. The flowers are rather inconspicuous, the 

 rays projecting but little above the apex of the enveloping involucral bracts, 

 which closely press together, forming a flower-head more or less flask- 

 shaped. 



At the time of flowering the involucral bracts spread somewhat at the 

 apex, and the tips of the flowers are a little more prominent. As the flowers 

 then wither, the bracts press closely together again and the head is closed. 

 As the seeds ripen the bracts die, and in drying bend outward and down- 

 ward, hugging the flower stem below, or they fall away. The seeds are 



