HARMFUL EFFECTS OF CROWDING UPON GROWTH 103 



toxic root secretions. Almost a century ago De Candolle, the French 

 botanist, suggested that the reason for the decrease in yield following 

 the continued growth of the same crop on the same soil is due to the 

 accumulation in the soil of harmful material given off by the growing 

 plants. Liebig apparently adopted this view for a time but aban- 

 doned it later, thinking that the observed benefits of crop rotation 

 were due to the different nutrient requirements of the crops rather 

 than to the accumulation of poisons in the soil. Pickering (191 7) 

 gives conclusive evidence that root excretions may have a toxic effect 

 upon growing plants. In his work he used mustard plants growing in 

 earth, on the surface of which rested a tray with a porous bottom, 

 with a large central walled opening through which the plants grew. 

 This tray held 5 inches of earth. The presence of such a tray made 

 practically no difference in the growth of the plants in the pot below, 

 even when the tray itself contained a growth of mustard plants, 

 providing their roots were kept out of contact with the soil of the 

 lower pot and that water from around the roots of the upper plants 

 was not allowed to reach the lower soil. When washings from the 

 upper growth were allowed to drain into the lower pot, carrying 

 leachings from the plants grown in the upper tray, growth of the 

 experimental seedlings was reduced to o.oi of that given in control 

 pots. Pickering found such results common and widespread, and 

 especially well shown by the effect of grasses on the growth of apple 

 trees. In summarizing all the evidence on the subject, Russell (1927) 

 concludes that, while a toxin can be shown to be present, the toxin 

 concerned'is not stable and is non-specific. 



CESSATION OF GROWTH IN BACTERIAL CULTURES 



The long-recognized failure of cultures of micro-organisms, such 

 as bacteria and molds, to continue growing indefinitely has been 

 attributed to three main causes: the exhaustion of foodstuffs, the 

 accumulation of metabolic wastes or specific "autotoxins," or the 

 limitations imposed by actual physical crowding. Henrici (1928) 

 gives a good summary of the present state of knowledge in this 

 field. 



The possible effect of physical crowding in limiting growth must 

 be excluded because much more dense growths can be obtained with 



