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 
as 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- 
(‘4 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 znvolucre. 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, 
Fic. 132. —Spike of Plantain and ot which th e dan d elion) th e 
Headof Red Clover:  «¢§ 8 8 Oe SS eee an 
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 
