44 



FLOWERS AND THEIR WORK 



A composite head. 



Cross-pollination of a Head (Clover). In a flower cluster called a 

 head, a closely massed cluster of little flowers as clover, cross-pollination is 

 usually effected by bumblebees which rapidly work from one flower to 

 another in the same cluster, inserting their tongues deep into the flower 

 cup. The butterfly shown in the illustration inserts its proboscis (seen 



curled up like a watch spring on the under- 

 side of the head) into the flower. 



Cross-pollination of a Composite Head. 

 - This flower cluster, so often mistaken 

 for a single flower, is found only in the 

 great Composite family, to which so many 

 of our commonest flowers and weeds 

 belong. The daisy, aster, goldenrod, and 

 sunflower are examples of the Composite. 

 The composite head is well seen in a 

 daisy or the sunflower. This head has an 

 outer circle of green parts. These parts 

 look like sepals, but in reality are a whorl 

 of leaflike parts. Taken together these 

 form an involucre. Inside the involucre 

 is a whorl of brightly colored, irregular flowers called the ray flowers. 

 They appear to act, in some instances at least, as an attraction to in- 

 sects by showing a definite color (see 

 the common dogwood, Cornus florida). 

 The flowers occupying the center of 

 the cluster are the disk flowers. Such 

 a flower examined under the hand 

 lens is found to be perfect. A care- 

 ful observer will find that the anthers 

 are united in a ring around the pistil. 

 This is a typical condition in the Com- 

 positse. The stamens ripen first and 



grow up around the stigma, which ripens later. The stigma splits (see a), 

 and pollen from another flower brought to its surface will germinate 

 there. 



Other examples. Many other examples of adaptations to 

 secure cross-pollination by means of the visits of insects might be 

 given. The mountain laurel, which makes our hillsides so beauti- 

 ful in late spring, shows a remarkable adaptation in having the 

 stamens caught in little pockets of the corolla. The weight of 

 the visiting insect on the corolla releases the anther of the stamen 

 from the pocket in which it rests, and the body of the visitor is 

 dusted with pollen. 



Section through composite head, 

 showing a disk flower (a), a ray 

 flower (c), and the involucre (d). 



