450 COMPETITION AND INVASION 



The majority of the grass plants were apparently seedlings of 

 the preceding autumn, and the plantains were young seedlings, 

 most of them an inch or less in height. A full-grown plantain 

 of this species occupies not less than 100 to 150 square inches, 

 so that of these alone more than 800 individuals were likely to 

 die of overcrowding. 



425. How overcrowding kills. Of plants grown too close 

 together many die and others are dwarfed (Fig. 349) and par- 

 tially or wholly fail to flower or seed. This is one of the first 

 lessons which the beginner in gardening learns, if he neglects 

 properly to thin out his beds. Corn grown in closely planted 

 drills for fresh fodder or ensilage makes few ears, and none of 

 these are perfect. The weakening or destruction due to over- 

 crowding results mainly from these three causes 1 : 



1. Insufficient light and heat for plants shaded by their more 

 vigorous neighbors, resulting in imperfect photosynthesis. 



2. Scanty water supply, because most of the water is absorbed 

 by the more vigorous root systems of the stronger individuals. 



3. Deficient supply of dissolved salts (nitrates, phosphates, 

 and so on), on account of their being largely consumed by 

 the stronger plants. 



426. Competition most fatal between similar plants. For 

 obvious reasons, plants of the same general form and mode of 

 growth usually interfere most with each other, and plants which 

 are decidedly unlike interfere less, or even in some cases benefit 

 each other. This principle is unconsciously followed, in many 

 instances, by farmers and gardeners, as in the case of lawns 

 sown with mixed grass seed, which produce a more perfect turf 

 than those sown with a single species of grass. So, too, pumpkins 

 are often planted in cornfields, and in southern Europe beans 

 are raised in vineyards, in the partial shade of the vines. 



If the interests of two or more kinds of plants occupying the 

 same area conflict little or not at all, this may be due not only 

 to their unlikeness of form or of requirements as regards light, 

 1 As far as terrestrial seed plants are concerned. 



