STRUGGLE FOR EXISTENCE 111 



the so-called key fruits of the maple or ash, and many others. 

 Corn, wheat, oats, etc., are indehiscent fruits. A grain is simply 

 a one-seeded fruit in which the wall of the ovary has grown so 

 close to that of the seed that they cannot be separated. Some 

 indehiscent fruits are light and carried by the wind, others may 

 be scattered by animals. 



Individual Project. Make a survey of your neighborhood to show 

 at least some examples in every method of dispersal discussed in this 

 unit. Make another classification of ways of dispersal, if you prefer. 



The struggle for existence. Those plants which provide best 

 for their young are usually the most successful in life's race. 

 Plants which combine with the ability to scatter many seeds over 

 a wide territory, the additional characteristics of rapid growth, 

 resistance to dangers of extreme cold or heat and to attacks of 

 parasitic enemies, inedibility, and peculiar adaptations to cross- 

 pollination or self-pollination, are usually called weeds. They 

 flourish in the sterile soil of the roadside and in the fertile soil 

 of the garden. By means of rapid growth they kill other plants 

 of slower growth by usurping their territory. Slow-growing plants 

 are thus actually exterminated. Many of our common weeds 

 have been introduced from other countries and have, through their 

 numerous adaptations, driven out other plants which stood in 

 their way. Such is the Russian thistle. First introduced from 

 Russia in 1873 in flaxseed, it spread so rapidly that it is now one 

 of the greatest pests in our Northwest. Water cress, introduced 

 in Australia by those who were fond of eating it in England, has 

 become such a pest that it chokes navigable rivers and has to be 

 dredged out frequently. 



Practical Exercise 4. Sum up all the ways in which weeds are successful 

 in the struggle for existence. 



Self-Testing Exercise 



Seeds of fleshy fruits are scattered by (1). Animals 



scatter seeds that possess (2), or (3). The wind 



carries the seeds of (4), (5) (6), and 



(7) great distances. Another common agent for dispersing 



