188 



FOUNDATIONS OF BOTANY 



spring from about the same point. This produces a 

 flower-cluster called the umbel (Fig. 130). 



199. Sessile Flowers and Flower-Clusters. Often the 

 pedicels are wanting, or the flowers are sessile, and then 

 a modification of the raceme is produced which is called 

 a spike, like that of the plantain (Fig. 132). The 

 willow, alder, birch, poplar, and many other common 

 trees bear a short, flexible, rather scaly spike (Fig. 

 131), which is called a catkin. 



The peduncle of a spike is -often so much short- 

 ened as to bring the flowers into a somewhat globu- 

 lar mass. This is called a head (Fig. 132). Around 

 the base of the head usually 

 occurs a circle of bracts known 

 as the involucre. The same 

 name is given to a set of bracts 

 which often surround the bases 

 of the pedicels in an umbel. 



200. The Composite Head. 

 The plants of one large group, 

 of which the dandelion, the 

 daisy, the thistle, and the sun- 

 flower are well-known members, bear their flowers in 

 close involucrate heads on a common receptacle. The 

 whole cluster looks so much like a single flower that it is 

 usually taken for one by non-botanical people. In many 

 of the largest and most showy heads, like that of the 

 sunflower and the daisy, there are two kinds of flowers, 

 the ray-flowers, around the margin, and the tubular disk- 

 flowers of the interior of the head (Fig. 133). The early 

 botanists supposed the whole flower-cluster to be a single 



FIG. 132. Spike of Plantain and 

 Head of Red Clover. 



