MULTIPLICATION IN PLANTS 



477 



=-^/ 



From the point of view of the species there are three factors 

 in seed dispersal that are of fundamental importance : ( i ) the 

 number of seeds scattered ; ( 2 ) the distance to which they are 

 carried; and (3) the final lodgment in a place favorable both 

 to germination and to later growth and development. 



We can see that the more seeds there are scattered, the better 

 are the chances that enough of them will find suitable lodgment 

 to replace the in- 

 dividuals that die 

 each year. On the 

 other hand, an ex- 

 cessive number of 

 seeds would be 

 wasteful and might, 

 under some circum- 

 stances, more than 

 make up for the ad- 

 vantage of numbers. 

 Thus, the orchids, 

 which produce tre- 

 mendous numbers 

 of very small seeds, 

 lose many ; only a 

 very small propor- 

 tion of them ever 

 develop into new 



plants. On the whole, the plants that depend upon the wind 

 to scatter their seeds seem to maintain themselves and to in- 

 vade new regions more successfully than those that depend 

 upon other agencies for scattering the new individuals. 



Many plants have their seeds distributed by currents of water 

 (streams of various sizes or ocean currents) or by wind cur- 

 rents acting on the water. Seed plants that grow in swamps 

 or ponds are commonly dependent upon water currents for the 

 dispersal of their seeds, but many seeds are also spoiled by 

 the water. 



®. 



d Q 



Fig. 199. Seeds scattered by birds 



Birds eat the fruit and discharge the indigestible seeds. 

 /, thistle; 2, mistletoe; 3, bird cherry; 4, red-osier 



dogwood 



