30 FLOWERS 



is the catkin in which the flowers, unisexual in t3'pical 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 as in Red Clover, then a head is formed. 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 stnacture known as the involucre. {Fig. 31.) 



B 



Fig. 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- 

 ercB, 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. (Fig. 32.) 



