846 



ECOLOGY 



FIG. 1173. A flowering 

 shoot of the yarrow (Achillea 

 Millefolium), illustrating the 

 massing of flowers into heads 

 (h), and the massing of heads 

 into a compact corymb; r, ray 

 flowers; i, involucre. 



be the more specialized, since they contrast 

 more sharply with the foliage, not only in 

 aspect but fundamentally, inasmuch as most 

 yellow petals resemble foliage leaves in hav- 

 ing plastids. 



In many flowers showiness is increased by the 

 presence of party-colored effects. Sometimes the 

 two halves or lips have different colors (as in Col- 

 linsia and Viola pedata bicolor), but more commonly 

 the variegation is due to spots or lines on a back- 

 ground of another color. In some plants with vernal 

 flowers (as Hepalica) a group of individuals may 

 exhibit a number of colors, varying from white 

 through pink to blue, thus greatly increasing the 

 showiness of the plant group as a whole. Often 

 flowers that are inconspicuous individually are so 



massed into compact inflorescences as to produce a showy 



effect; such a condition is seen in the umbellifers and even 



more in the composites, where the inconspicuous central or 



disk flowers often are surrounded by showy outer or ray 



flowers, giving the effect of a large simple flower (fig. 1173). 



The inflorescences of Hydrangea consist similarly of incon- 

 spicuous central and of showy outer flowers, the latter being 



sterile. In some plants the calyx is the showy organ (as in 



Abronia and Mirabilis), and in some species of Castilleja. 



Euphorbia, and Monarda the bracts, or even the upper 



leaves, are much showier than are the relatively insignificant 



flowers. In some dogwoods the involucre is much showier 



than are the flowers, and in the willows where there is no 



perianth, the staminate catkins often are showy by reason 



of the conspicuous stamens. 



Zygomorphy. Zygomorphy or irregularity in the corolla 



often adds to the conspicuousness of flowers. Many flowers 



are labiate or lipped (as in the mints and the legumes), the 



lower lip commonly protruding farther than the upper (fig. 



1174); the culmination of lip development and zygomorphy 



is found in the orchids, whose flowers are noted for their 



bizarre shapes. The projecting lower lip is of obvious 



advantage as a landing place for pollinating insects, notably 



the bees. Often, as in the flowers of many legumes, the 



weight of the insect presses down the lip sufficiently to 



expose the anthers and the stigma. Floral lips are of no 



advantage for the hawk moths and for similar insects, which 



hover before the flowers without alighting. As a class, acti- 



nomorphic flowers are erect, contrasting with the generally 



FIG 1174. Flow- 

 ers of Coleus, illus- 

 trating zygomorphy; 

 the calyx (c) and the 

 sympetalous corolla 

 (c') are bilabiate, the 

 latter having an as- 

 cending upper lobe (b) 

 and a descending boat- 

 shaped lower lobe (a), 

 from which the sta- 

 mens and the style 

 are partially exserted; 

 the lower lobe of one 

 flower (/) is held to 

 one side, so as to show 

 more clearly the up- 

 turned style (t) with 

 its two-lobed stigma 

 (g) and the four sta- 

 mens (5). 



