216 EVOLUTION OF PLANTS 



the great family of Compositse is the highest of all. 

 The latter is the largest family of plants and shows 

 extreme specialization of the floral structures in the ex- 

 tensive cohesion of the parts, which extends to the sta- 

 mens as well as the other parts of the flower. The 

 flowers, as is well known, are aggregated in dense heads 

 surrounded by bracts which give the whole inflorescence 

 the appearance of a single flower (Fig. 53, E, G). This 

 is especially so in such forms as the asters and daisies, 

 where the outer flowers have the corolla large and flat- 

 tened, so that each of these " ray-florets " looks like a 

 single petal. In many genera these outer flowers are 

 destitute of stamens and sometimes the pistil is also 

 abortive, and the ray-florets serve simply to make the 

 inflorescence conspicuous. There are many interesting 

 transitions between the lower Compositse, where all the 

 flowers of the inflorescence are alike, and those in which 

 the ray-florets are entirely sterile. 



The type of inflorescence developed in the Compositse 

 seems to have been particularly effective, as these plants 

 are notoriously prolific. The actual number of seeds is 

 not excessively large as compared with many other 

 plants ; but each individual flower almost always suc- 

 ceeds in ripening its seed, and the one-seeded fruits are 

 usually provided with most efficient means of transpor- 

 tation. One has but to think of the legions of common 

 Composites, — daisies, sunflowers, thistles, burdocks, 

 dandelions, and many others of our commonest and 

 most troublesome weeds, — to realize how well fitted 

 these plants are to hold their own in the struggle for 

 existence. 



