DISPERSAL AND GERMINATION OF SEEDS 171 



In the touch-me-not (" wild balsam ") a strain is devel- 

 oped in the growing wall of the seed-vessel, so that at 

 rupture, which may be brought about by a slight pressure, 

 the pieces suddenly curl up and throw the seeds. This 

 mechanical method may be regarded as the poorest of all 

 the methods of dispersal, for at the very best a seed-vessel 

 can discharge its seeds only a very short distance. 



A more effective method of dispersal and a much more 

 common one is by means of currents of air. This means that 

 seeds or fruits must be very light, or that they must develop 



FIG. 140. Winged seed of Bignonia (a relative of catalpa). After STRASBURQEK. 



special appendages to aid in their flight. Among the most 

 common appendages are tufts of hair and what are very 

 naturally called " wings." For example, plumes and tufts 

 of hair are developed by the seed-like fruits of thistle and 

 dandelion (Figs. 135 and 136), and by the seeds of milkweeds 

 (Fig. 137) and fire weeds (Fig. 138) ; while wings are developed 

 by the fruits of maples (Fig. 139) and elms, and by the seeds 

 of catalpa (Fig. 140). An interesting modification of the 

 wind-dispersing habit is exhibited by the "tumble weeds" or 

 " field rollers" of the western plains and other level stretches 

 (Fig. 141). These plants are profusely branching annuals 

 with a small root system in light or sandy soil. When the 

 work of the season is over, and the rootlets have shrivelled, 



