148 THE SECOND BOOK OF BOTANY. 



When part of the florets are imperfect , the head 

 is said to be heterogamous. 



Flower-heads are discoid when destitute of ray 

 florets. 



EXERCISE L. 

 Characters of the Compositce. 



Dandelions, daisies, dahlias, thistles, etc., we see, 

 are composed of many florets, enclosed in a calyx-like 

 involucre. Plants of this kind have, therefore, been 

 named composite from the compound, or composite, 

 nature of what, to the untaught, seems a single 

 flower. They form one of the most numerous, and, 

 at the same time, one of the most natural and perfect 

 families in the vegetable kingdom. There are about 

 nine thousand different species included in it. They 

 are found in all countries and climates. About -J- of 

 the plants of North America, and |- of all tropical 

 plants, belong to it ; indeed, from % to -fa of all the 

 plants in the world are of this order. 



Now, why is this order said to be very natural ? 

 Why, for instance, is it a more natural group than the 

 rose family ? If examples of all these nine thousand 

 species were brought together, they would be seen to 

 have one conspicuous and many important characters 

 in common. In every one of them the inflorescence 

 is a dense head, enclosed in a more or less compact 

 involucre. But, when you have collected all the 

 members of the rose family, you do not see so many 

 features common to all, nor any marked one which 

 stamps them as similar. On the contrary, in all their 



