296 ECOLOG y. 



462, The wild lettuce. A related species, the wild lettuce 

 (Lactuca canadensis) 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 pro- 

 jecting 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 some- 

 what 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 downward, hugging 

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

 exposed. The dark brown achenes stand over the surface of 

 the receptacle, each one tipped with the long slender beak of 

 the ovary. The " pappus," which is so abundant in many of 

 the plants belonging to the composite family, forms here a 

 pencil-like tuft at the tip of this long beak. As the involucral 

 bracts dry and curve downward, the pappus also dries, and in 

 doing so bends downward and stands outward, bristling like the 

 spokes of a fairy wheel. It is an interesting coincidence that 

 this takes place simultaneously with the pappus of all the seeds 

 of a head, so that the ends of the pappus bristles of adjoining 

 seeds meet, forming a many-sided dome of a delicate and 

 beautiful texture. This causes the beaks of the achenes to be 

 crowded apart, and with the leverage thus brought to bear upon 

 the achenes they are pried off the receptacle. They are thus in 

 a position to be wafted away by the gentlest zephyr, and they 

 go sailing away on the wind like a miniature parachute. As 

 they come slowly to the ground the seed is thus carefully 

 lowered first, so that it touches the ground in a position for 

 the end which contains the root of the embryo to come in con- 

 tact with the soil. 



463. The milkweed, or silkweed. The common milkweed, 



