1566 



A TEXTBOOK OF THEORETICAL BOTANY 



Fig. 1428. — Siegesbcckia nrientalis. Fruiting capitula covered with 



intensely adhesive bracts. 



line of evolution has developed them from devices for wind dispersal. 



Indeed many of the typically wind-dispersed pappus fruits are also able to 



cling to animals as an alternative means of trans- 

 port and the change over to this mode of dispersal 

 involved relatively little alteration of structure. 

 The third point is that geographically the greatest 

 number of adhesion devices are found in open, 

 grassy country, where herds of wild ungulates 

 roamed in former ages, and where man has now 

 substituted his grazing stocks. Relatively few are 

 in forest or desert plants. 



Simple adhesion either by means of sticky 

 secretions or simply by wetting is a further im- 

 portant means of dispersal. Among the former 

 we may mention Siegesbeckia orientalis, a small 

 Composite with sheathing bracts covered with 

 glands of so adhesive a nature that the seed- 

 bearing capitulum can be picked up off the ground 

 merely by touching it with the point of a needle 



^ r,. , , • (Figs. 1428 and 1420). 



Fig. 1^29.— Siegesbeckia ° ^ c j • t^ t m • r 



orientalis. Capitulum be- hticky calyces are lound m Verbena offiariaus 



ing picked up by a single (Vervain) and in the genus Plumbago. Both are 

 strand of the adhesive .. . . 



matter. readily adhesive to long hairs and to clothes. 



