FBTTITS AND SEEDS. 257 



287. Dispersal of Seeds and Fruits. Such a plant 

 as a Cherry tree may produce thousands of fruits and fertile 

 seeds in a single season, although none of the latter may 

 succeed in establishing themselves as young seedlings beneath 

 the shade of the parent tree. Even if the parent plant is 

 cut down, or dies at the end of the season, it can hardly 

 make room for more than one or two of its offspring. 

 Hence it is of the utmost importance that the seeds should 

 be afforded some chance of distributing themselves, and 

 reaching localities suitable for the development of new 

 trees. Birds act as the dispersing agents in this case, fre- 

 quently carrying the fruit to a distance, and rejecting the 

 hard stone which protects the seed. 



Even in the case of annual plants, the dispersal of the 

 seeds is of great importance, for it is evidently impossible that 

 the thousand or so seeds which a single Sunflower may pro- 

 duce can germinate and become healthy seedlings on the same 

 area that the adult plant occupied. The means of dispersal 

 adopted by different plants vary widely, and are frequently 

 such as to secure the almost ubiquitous distribution of some 

 plants. The chief agencies by which dispersal is secured are : 

 (1) wind, (2) water, (3) animals, and (4) explosive or 

 ejection mechanisms in the fruit itself. 



288. Dispersal by the Wind is facilitated by the 

 minuteness or lightness of the seeds or fruits, and by the 

 presence of tufts of hairs or wing-like membranes which 

 increase the surface exposed to the wind without appreciably 

 adding to the weight. It is only in the case of dehiscent 

 fruits that mechanisms for wind-dispersal are borne by the 

 seed ; closed fruits and the segments of splitting fruits are 

 themselves distributed and possess contrivances for dispersal, 

 while the seeds have none and are carried inside the fruit. 

 The seeds of Orchids are so small and light that, when they 

 are set free by the dehiscence of the capsules, they are 

 freely blown about by the wind, hence the epiphytic tropical 

 Orchids establish themselves upon the trunks of trees. 



When the seeds are larger and heavier, the fruit frequently 



opens in such a manner that the seeds can only escape a few 



at a time and are jerked out when strong winds rock the 



capsules to and fro. This censer mechanism is seen in 



S.B. 17 



