SEED DISTRIBUTION 



169 



Evidently no kind of flowering plant actually increases at 

 any such rate as has just been suggested, or it would soon 

 crowd most others out of existence. The means by which the 

 unlimited multiplication of any one 

 species is prevented are lack of ex- 

 tremely rapid and thorough means of 

 disseminating the seeds, multiplication 

 of the insect and plant enemies of the 

 species (a factor which is often not 

 very important), and over-crowding or 

 competition with other plants of the 

 same or of different species. 



161. Competition as a check on in- 

 crease. No one can realize just what 

 competition among plants means un- 

 less he makes some careful out-of-door 

 studies of plants growing under condi- 

 tions of great overcrowding. A por- 

 tion of a grainfield too thickly sown, 

 a very weedy bit of garden soil left 

 to itself for the whole growing season, 

 or a piece of recently cleared forest 

 in which coppice growth is starting 

 from old stumps, or where seedling 

 trees are springing up in great num- 

 bers any one of these will teach a 

 most important lesson. The writer 

 has found wild-black-cherry seedlings, 

 to the number of more than 100 to 

 the square foot, beginning to grow in 

 the spring under a large wild-cherry 

 tree. As the parent tree was in thrifty 



condition and its top was nearly 30 feet in diameter, there 

 might have been some 70,000 seedling cherries every year 

 killed by crowding and shade under this one tree, although, 

 in fact, there were never so many as this. 



FIG. 150. Cockleburs 



Tins troublesome weed often 

 grows along a path, where 

 men carry the seeds back and 

 forth in their clothing, and 

 animals in their hair or fur 



