170 



INTRODUCTION TO BOTANY 



Competition sometimes results in killing outright most of 

 the competing plants ; sometimes it only renders them dwarf- 

 ish and incapable of producing many flowers or seeds. 



The means by which the successful individuals weaken or 

 kill their neighbors are mainly 



1. Overshadowing, resulting in deficient photosynthesis in 

 the shaded plants from lack of light. 



2. Robbing the defeated plants of water. 



3. Robbing them of soluble salts, such as nitrates and phos- 



-.^ phates of the soil. 



The deprivation 

 of sufficient water 

 and salts interferes 

 with the nutrition 

 of the overcrowded 

 plants and may soon 

 stop their growth. 



162. Mechanisms 

 for distributing seeds. 

 Many seeds, such as 

 those of the catalpa, 

 the milkweed, and 

 the willow, have 

 wings or tufts of 

 down which insure 

 their being carried 



considerable distances by the wind. Tufted fruits, such as those 

 of the thistle and the dandelion (fig. 147), are familiar to most 

 people. Sometimes the plant retains the seeds or fruits for 

 months after they are ripened, and thus secures their gradual 

 dispersal. The globular clusters of fruits of the sycamore 

 (fig. 148) remain on the tree during the entire winter many of 

 them even till the new crop of leaves has appeared in the spring. 

 Frequently the pod, or capsule, is so constructed that it 

 opens at the top (fig. 149) and scatters seeds whenever it is 

 swung to and fro by the wind or jostled by a passing animal. 



FIG. 151. Fruit of the wild black cherry, a very 

 valuable timber tree 



The seeds, which are hard and indigestible, are dis- 

 seminated mainly by birds. One half natural size 



