164 



Some Factors in Plant Co/npetitlon 



The actual effect of light competition on individual plants is clearly- 

 shown by the dry weight at different points of the crowded square. (Fig. 9. ) 



The plants in the outer rank are less subject to light competition 

 than any of the others, as they are quite free and exposed on one side; 

 most of these are fairly heavy, 14 out of 28 plants (50 per cent.) being 

 above the upper limit of weight of the interquartile range, and none 

 below it. The second rank suffers from the proximity of its healthy 

 neighbours of the outer row, and is also flanked by others on the inner 

 side, with the result that only 10 per cent. (2 out of 20) are above the 

 upper limit and 35 per cent. (7 out of 20) are below the lower limit of 

 the interquartile range. The third rank is worse still, with no heavy 

 plants, but with 58 per cent. (7 out of 12) below the lower limit of range, 

 while the innermost core of the square consists of four plants suffering 

 most severely from the light shortage, 3 of them (75 per cent.) being 

 below the lowest limit and the fourth not far above it. These results 

 show more clearly when tabulated (Table XII). 



The depreciation in growth caused when even so small a number of 

 plants as 64 are crowded is thus very great, and it is obvious that if the 

 number were increased the total depreciation would be greatly aug- 

 mented, as all the plants within the outer row or two of the mass would 

 feel the effect of competition most severely. It is not a question of 

 competition for food by the roots as every plant had its own individual 

 supply which was never allowed to run short, but the competition for 

 light prevented the inner plants from making adequate use of their 

 mineral resources, and less dry matter was produced in consequence. 



The physiological effect of this struggle for light finds morphological 

 expression in the relations between the roots and shoots of the competing 

 plants. In every case a full quantity of nutrients was supplied to the 

 roots, but the amount of light available to carry on elaboration of sap 

 and assimilation of carbon-dioxide in the leaves varied with the indi- 

 vidual plant. When overcrowding occurred this implied a lessened 



