30 FLOWERS 
is the catkin in which the flowers, unisexual in typical cases, 
usually have scaly bracts instead of a true perianth, and the 
whole cluster falls after fruiting. Catkins are typical of Poplars, 
Willows, Hickories, and Birches. When the raceme is so short 
that the compact mass of flowers form a more or less rounded 
cluster asin Red Clover, then a head isformed. In the Composites 
there is the special kind of head which is the most highly organ- 
ized of all flower clusters. The flowers besides often being differ- 
entiated into two kinds are so compactly arranged as to form a 
cluster resembling a single flower and the cluster is surrounded by 
bracts, which form a structure known as the involucre. (Fig. 31.) 
Fic. 32. — A, Corymb of one of the Cherries. B, umbel of a species 
of Onion. 
In contrast to the spike there are those raceme-like clusters 
in which the flowers have long stalks, as in the typical panicle, 
where the cluster is loosely branched. When the portion of stem 
to which the flowers are attached is short and the stalks of all of 
the flowers are so elongated as to bring all of the flowers to about 
the same level then a corymb results. A further modification in 
which the portion of stem to which the flowers are attached is so 
short that the flower stalks appear to be of the same length and 
attached in a circle around the stem results in the umbel, the form 
of cluster characteristic of the Parsley Family, called Umbellif- 
ere, on account of the character of the flower cluster. Of this 
family the Parsnips, Carrots, and others are common. The um- 
bel is also common among the Milkweeds. Umbels may be 
simple or compound, that is, so branched as to be composed of 
a number of small umbels. (Fzg. 32.) 
