THE DISPERSION OK SKKDS AND SI'OKES. 121 



hairs secretinj^ viscid matter available for dispersion. 

 Sticky secretions also occur in connection with the 

 seed or adjacent parts in Pisonia acideata, Boerhavia 

 scandens, B. erecta, Adenostemma, DryinaHa, Sieges- 

 beckia, and others. Adhesive fruits will more easily 

 succeed in attaching themselves to a smooth surface 

 than hooked seeds can, so that hairless mammals 

 and even rej)tiles will be available for their dis- 

 persion. These contrivances by which seeds and 

 fruits fasten themselves to animals remind us of 

 the sticky spores of Sphcerobolus, and the adhesive 

 disks by which the orchid fastens its pollen-masses 

 to the head of an insect. 



. The contrivances which favour wind-dispersion 

 shade insensibly into those Avhich promote distribu- 

 tion through animal agency, and it is easy to see 

 how the former might be transformed into the 

 latter. Besides its pappus, so perfectly adapted for 

 wind-dispersion, the dandelion has, as we have seen, 

 a number of projections on the solid body of the 

 seed; and a scabrid surface is very common on 

 plumed seeds. The fruit of the dandelion, if the 

 pappus be removed, will even adhere to cloth, but 

 there can be no need for the help of animals Avhere 

 the fruit is so admirably adapted for wind-transport. 

 It is of advantage to a wind-borne seed if, when it 

 alights, it has some provision by which it can anchor 

 itself and so come to rest. What the graj^ple is to the 

 balloonist these protuberances are to a wind-driven 

 seed. At the same time, these projections will cause 

 the seeds readily to adhere to any animal that 

 touches them, and in coui'se of time this mode of 

 dispersion might supersede the other. Should this 

 ever happen to the dandelion, the pappus would 

 disappear, leaving the fruit with an awn-like beak 

 which might either persist as a skewer or be dis- 

 carded as superfluous. At the same time, we should 

 iBxpect the booklets to increase in size and efficiency, 

 so as to adapt the fruit more perfectly to animal 

 transport. It is at least highly probable that some 



