188 



FOUNDATIONS OF BOTANY 



1 



spring from about the same point.. This produces a 

 flower-cluster called the umhel (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 Bed Clover. 



