STUDIES OF SEED-PLANTS 



211 



bracts correspond to leaves on stems beneath ordinary flowers, and 

 not to the calyx. The white petal-like structures surrounding 

 clusters of dogwood flowers are also examples of bracts. 



Any one of the numerous flowers in a dandelion flower-head has 

 the structure shown in Fig. 67, a. The tuft of bristles or hairs (called 

 pappus) and the ring below the hairs represent the calyx. The sur- 

 rounding corolla is tubular at its base and the upper part is a 

 flattened strap-like structure 

 called a ligule. Five petals 

 are united in this peculiar 

 corolla. It is evident then 

 that the petal-like structures 

 which first attract attention 

 when one casually looks at 

 a dandelion flower-head are 

 simply the strap-like exten- 

 sions from as many corollas. 

 The five stamens are united 

 at their anthers, and protrud- 

 ing up through the center of 

 this ring of stamens is the 

 style, which is forked. The JT IG 57 

 position of the ovary is the 

 same as that shown at the 

 bottom of Fig. 67, a, c. 



Sunflower, Aster, Calen- 

 dula, Coreopsis, Zinnia, 

 Marigold. (L) These are 

 common flower-heads with 

 structure similar to that of the dandelion, except that only the flowers 

 at the margin of the head have the strap-like rays. All the other 

 flowers in the heads have the same structure; they are tubular 

 flowers without rays. The flowers without the straps are called disc- 

 flowers, those with straps are ray-flowers. It is evident that the 

 conspicuousness of sunflowers and their relatives named above is 

 due largely to the large and colored rays of the marginal flowers. 



Flower-heads of Common Thistles, Burdock, Centaurea, Tansy, Iron- 

 weed. (D) These have no strap-like rays on any of the corollas 

 in the head. In thistles all flowers are alike and similar to the 

 disc-flowers of the sunflower head. In centaurea the marginal 

 flowers are enlarged and without stamens and pistils (i.e., they 

 are sterile.) 



a o v c 



Composite flowers of Arnica, a, 

 ray-flower ; b, disc-flower consisting of a 

 calyx with bristles, a tubular corolla 

 with five lobes, style divided into two 

 stigmas, stamens united in ring around 

 the style ; c, longitudinal section of b. 

 Some composites have stamens in ray- 

 flowers (a). (From Strasburger .) 



