THE HIGHER DICOTYLEDONS. 375 



spread by means of the underground branches and to its 

 very effective method of choking off competing plants by its 

 large leaves. 



372. The Composite Family (Compositae) is the largest 

 among flowering plants, and one of the most successful and 

 widely distributed. There are about 800 genera (40 in 

 Britain) and 11,000 species (115 in Britain), i.e. over 10 

 per cent, of the total number of species of Flowering Plants ! 

 There are nearly 1300 species of one genus (Senecio) alone. 

 It is easy to see why this family is such an aggressive and 

 successful one. What are the advantages of having (1) 

 numerous small flowers grouped together in a compact flat- 

 topped head, (2) the outer flowers often more conspicuous 

 than the inner ones, (3) honey in most cases accessible to 

 even short-tongued insects, (4) in most cases small light 

 fruits provided with a pappus ? 



Examine all the Composites you meet with and find out all 

 you can about the arrangement and structure of the heads 

 and of the individual flowers. How do the heads and flowers 

 of Cornflower and Knapweed ("Hard-heads") differ from 

 those of our three types ? What other Composites resemble 

 (1) the Dandelion in having all the flowers ligulate, (2) the 

 Daisy in having the outer flowers ligulate and the inner ones 

 tubular, (3) the Cornflower (" Bluebottle ") and Knapweed 

 in having all the flowers tubular ? How do the outer flowers 

 of the Cornflower-head differ from the inner ones? What 

 common Composites have small heads densely massed to- 

 gether to form a flat-topped corymb of heads, and what are 

 the advantages of this arrangement? In each case study 

 the biology of the inflorescence and of the individual flower, 

 in the way we have studied the Dandelion and Daisy. 



373. Mechanisms in Compositae. The general struc- 

 ture of the flower is remarkably uniform throughout this huge 

 family. As regards the number of flowers in a head, we get 

 every transition from the single-flowered heads of G-lobe 

 Thistle (Echinops, cultd.), through the few-flowered ones of 

 Hemp Agrimony, etc., to the huge heads of Sunflowers with 

 hundreds of flowers. 



