338 



A TEXTBOOK OF BOTANY [Cn. VI, 11 



STERILE 

 y FLORETS 



to, a large family of plants, the Umbelliferse. Both 

 corymbs and umbels also become branched or compounded. 

 Still more advanced in evolutionary rank are those clusters 

 in which there is found a division of labor 

 with respect to the functions of reproduction 

 and conspicuousness. In some clusters the 

 conspicuousness which shows the flower to 

 insects is given by bracts greatly developed, 

 as with the Calla and Jack-in-the-pulpit, 

 where the single showy bract or SPATHE acts 

 functionally like a corolla, leaving only the 

 function of pollination to the little incon- 

 spicuous flowers arranged on a fleshy spike 

 calledajsPADix (Fig. 236). BfacTsUso form 

 the showy parts of the flat-topped clusters 

 of comparatively inconspicuous flowers in 

 Poinsettia and Flowering Dogwood. Still 

 more highly developed are those clusters in 

 which this division of function occurs be- 

 tween the flowers themselves. Thus, in the 

 wild Hydrangea and its relatives, the inner 

 flowers of the flat-topped compound cyme 

 remain inconspicuous, and the showiness of 

 the cluster is due to the petals of the outer- 

 most flowers which have developed very 

 greatly (Fig. 237), losing entirely in the 

 process their reproductive parts. It is these 

 outer NEUTRAL flowers which have been de- 

 veloped in cultivation to form the fine great 

 showy pyramidal clusters (thyrsi) of our lawn 

 Hydrangeas. This arrangement reaches its 

 highest development in the family Com- 

 posite, where, in forms like the Sunflower, 

 the outer row of the flowers (the so-called RAY FLOWERS) in 

 the dense, flat-topped cluster develop greatly their corollas 

 which make the whole showy parts of the head, but lose their 



BASE 

 OF 



SPATHE 



FIG. 236. 

 The spadix, with 

 flowers, of an 

 Arum ; the large 

 showy spathe is 

 removed. (From 

 Cavers.) 



