CHAP, iv.j REPRODUCTION OF PLANTS. 



153 



the down of poplar-seeds, for example, having quite a 

 different origin. It is interesting to note that the 

 important features mentioned as characterizing the mem- 

 bers of the order Compositae the reduction in the size 

 of the flowers and their concentration into dense heads ; 

 the involucre as a protective organ ; and the pappus, a 

 dispersive organ, are not new factors added, but simply 

 modifications of organs previously existing ; and within 

 the order we have every transition 

 from the older and original function 

 of the parts enumerated above to 

 that of their complete modification ; 

 if this were not the case, it is very 

 doubtful whether we should ever 

 have been able to state that the 

 pappus is a modified calyx. This 

 is what is meant by the term evolu- 

 tion. The best proof that one or 

 more departures from the ordinary 

 track, evolved by a group of plants, 

 are of real service in enabling 

 the organisms concerned to better 

 maintain their position in the 

 struggle for existence, is shown by 

 their geographical distribution, and in this respect the 

 members of the natural order Compositse stand out pro- 

 minent. In addition to the points of advantage already 

 indicated, composite plants produce very minute fruits 

 that are easily carried by the wind and are thus dispersed 

 over wide areas ; their protective arrangements are also 

 well developed, and there is what may be termed an all- 

 round provision and tendency to meet circumstances and 



Fig. 43. Fruit of 

 dandelion surmounted 

 by a stalked pappus. 

 (Slightly magnified.) 



