HOW FLOWEBS CLUB TOGETHER. 157 



of five petals, united into a tube. Then come 

 the five united stamens, and the pistil with its 

 divided stigma. This is the simplest and central -, 

 form of composite, from which the others are 

 descended with various modifications. 



To this central type belong a large number of 

 well-known plants, both useful and ornamental, 

 though more particularly deleterious. Among 

 them may be mentioned the various thistles, 

 such as the common thistle, the milk thistle, the 

 Scotch thistle, and so forth, most of which have 

 their involucres, and often their leaves as well, 

 extremely prickly, so as to ward off the attacks 

 of goats and cattle. The burdock, the artichoke, 

 the saw-wort, and the globe-thistle also belong to 

 the same central division. Among these earlier 

 composites, however, there is one group, that of 

 the centauries, which leads us gradually on to 

 the next division. Our commonest centaury in 

 Britain (known to boys as hardheads) has all the 

 florets equal and similar, and looks in the flower 

 very much like a thistle. But one of its forms, 

 and most of the cultivated garden centauries, 

 have the outer florets much larger and more 

 broadly open than the central ones, so that they 

 form an external petal-like row, which adds 

 greatly to the attractiveness of the entire flower- 

 head. Of this type, the common blue cornflower 

 is a familiar example. Clearly the plant has 

 here developed the outer florets more than the 

 inner ones in order to make them act as extra 

 special attractions to the insect fertilisers. 



The more familiar type of composites so much 

 cultivated in gardens carries these tactics a step 



