908 ECOLOGY 



only through the agency of seeds, the situation is different, since the 

 same individual produces seeds a number of times. The adequacy 

 of seed protection is well illustrated by the abundant annual recur- 

 rence of such weeds as the ragweeds, pigweeds, purslane, and Russian 

 thistle. 



Seed protection in relation to animals. Many seeds are used as food 

 by herbivorous animals. Often, as in the nuts that are eaten by squirrels 

 and in the many small seeds that are eaten by birds, the protective coats 

 are insufficient to give adequate protection, the survival of the species 

 depending upon those seeds that chance not to be eaten. The likeli- 

 hood of such survival is not so slight as it might seem, since most species 

 produce many more seeds than would commonly be eaten, and many 

 seeds fall to the ground and become hidden by leaves. The seeds of 

 edible fruits might be thought to be in especial danger, but in most cases 

 they pass through the digestive tracts unharmed. The smooth and 

 slippery surfaces and the pointed ends of most such seeds make it prob- 

 able that they will be swallowed whole rather than masticated, and the 

 thick and hard testa prevents the destructive action of digestive juices 

 upon the living contents. Sometimes the seeds, as in the grape, are en- 

 closed by a mucilaginous pulp that is likely to be swallowed whole, and 

 sometimes they are protected by special structures, such as the car- 

 tilaginous layers within the apple. 



The vitality of seeds. The amount of protection exhibited by seeds 

 is shown in no other respect so well as by their remarkable longevity. 

 While some seeds (as in the willow and the cacao) die unless they 

 germinate almost immediately, most seeds retain their viability for 

 several months or even years, and a few may remain alive for many 

 years. 



There is a popular belief in the possession of extreme longevity by 

 certain seeds. For example, it often is asserted that the reason for the 

 development of a totally new kind of vegetation when a forest is cleared 

 is that seeds which have lain dormant for years or even for centuries 

 then for the first time have a chance to germinate; a much simpler ex- 

 planation, however, is found in the ease of seed dissemination. Many 

 people have believed that wheat buried many centuries ago with the 

 Egyptian mummies has germinated in recent times when properly 

 planted. While stories of this character are without foundation, never- 

 theless it is true that under proper conditions certain seeds may remain 

 alive for many years. Probably the longest-lived seeds are those of the 



