Chapter XXXIX. 



Dandelions, Ragweeds and Thistles. 



The last and highest family comprises the plants known as 

 Composites. It includes some 11,000 species of herbs and 

 shrubs, with a very few trees, well distributed over all regions 

 of the world, and is characterized by the aggregation of the 

 flowers into composite heads. These heads are subtended by 

 groups of bract-leaves to which is given the name of involucre. 

 The involucre is a green, closely packed series of scales, such 

 as may be seen below the head of a sunflower or thistle. In 

 some plants, such as the burdocks or cockleburs, the involucre 

 produces bur-like, hooked or barbed groups of bristles, by 

 means of which all the fruits of a head are distributed as a unit. 

 There are two main series of composite plants, one of which 

 is distinguished by the presence of a milky juice, while in the 

 other no milk exudes on the breaking of the stems or foliage. 

 Dandelions, lettuce, sow-thistles and chicory serve to illustrate 

 the first series, while asters, goldenrods, burdocks, sunflowers, 

 thistles, cockleburs and ragweeds are examples of the second. 

 The composites with milky juice develop what are known as 

 strap-shaped corollas for all the flowers of a head. Such strap- 

 shaped corollas may be conceived to arise by the splitting of 

 a tubular corolla like that of a honeysuckle down one side, 

 near to the fruit-rudiment. Very often, as in dandelion flowers, 

 five little notches will be found at the edge or tip of the strap. 

 These represent the five fused petals of the original tubular 

 flower. Those members of the composite family which have 

 no milky juice produce, in many instances, at the margin of 

 the head, the strap-shaped flowers ; but some, and often all the 

 flowers in a head are tubular, like miniature honeysuckle flow- 

 ers. The sunflower or the daisy furnishes an example of a head 

 in which strap-shaped flowers are produced laterally and tubu- 



