428 RELATION TO ENVIRONMENT. 



of inflorescence, and the lengthening; of the lower, lateral flower 

 peduncles so that the flower cluster is more or less flattened on 

 top. This represents the simple corymb. A compound corymb 

 's one in which some of the flower peduncles branch again form- 

 ing secondary corymbs, as in the mountaiH ash. It is like a 

 panicle with the low'er flower stalks elongated. 



823. The umbel. — The umbel is developed from the raceme, 

 or corymb. The main flower-shoot remains very short or unde- 

 veloped with several flowers on long peduncles arising close to- 

 gether around this shortened axis, in the form of a whorl or clus- 

 ter. Examples are found in the milkweed, water pennywort 

 (Hvdrocotvle), the oxheart cherry, etc. A co)n pound umbel is 

 one in which the peduncles are branched, forming secondary 

 umbels, as in the caraway, parsnip, carrot, etc. 



824. The spike. — In the spike the main axis is long, and the 

 soUtary flowers in the a.xils of the bracts are usually sessile, and 

 often very much crowded. The plaintain, muUein (fig. 422), 

 etc., are examples. The spike is a raceme, only the flowers are 

 sessile and crowded. In the grasses the flower cluster is branched, 

 and the branchlets bearing a few flowers are spikelcts. 



825. The head. — When the flower axis is very much short- 

 ened and the flowers crowded and sessile or nearly so, forming a 

 globose or compressed cluster, it is a head or capi.ulnm. The 

 transition is from a spike by the shortening of the main axis, as 

 in the clover, button bush (Cephalanthus), etc., or in the short- 

 ening of the peduncles in an umbel, as in the daisy, dandelion, 

 and other composite flowers. In these the head is surrounded 

 by an involucre, which in the young head often envelopes the 

 mass of flowers, thus affording them protection. In some other 

 composites (Lactuca, for examjjle) the inwilucre affords pro- 

 tection for a longer period, even while the seeds are ripening. 



826. The spadix. — W hen the main axis of the flower cluster 

 is fleshy, the spike or liead forms a spadix, as in the Inthan tur- 

 nip, the skunk cabbage, the calla, etc. The spathx is usuallv 

 more or less enclosed in a spallie, a somewhat sirap-shaped leaf. 



S27. The catkin. — A spike which is usually cadui^ous, i.e.. 



