I 



Winifred K. Brenciiley 169 



in nitrogen content was observed throughout. In the outer rank, 

 though, in which ear formation was further advanced than in the interior 

 of the square the percentage of nitrogen in the grain was higher than 

 the average (.'5-02 per cent.) and in the straw was lower (2-08 per cent.) 

 thus following the normal law. 



The above experiment with spaced and crowded plants proves con- 

 clusively how potent a factor is light competition in determining the 

 growth of plants. Under ordinary conditions of cultivation overcrowding 

 inevitably reduces the food supply of the individual plants, and, as was 

 shown in the early part of this paper, this itself sets a limit to the amount 

 of possible growth. This is widely recognised, but the effect of deficient 

 light supply is less self-evident and therefore less taken into account. 

 When plants are crowded in the field the root range is less circumscribed 

 than when they are grown in pots and the available food supply is less 

 restricted. The harmful effect of overcrowding under such circumstances 

 is therefore probably due less to deficient nutrition than to light starva- 

 tion which can occur, as shown above, even when a full and abundant 

 food supply is assured. Further work on this point is needed, especially 

 with regard to broad leaved plants in which the degree of overshadowing 

 of one plant by another is accentuated, and in which possibly the factor 

 of light competition is therefore even more important than with such 

 narrow-leaved plants as barley. 



SUMMARY. 



1. The mutual action of one plant on another when grown in 

 juxtaposition, usually known as competition, is a very complex phe- 

 nomenon. Among the factors which come into play are competition for 

 food, water and light, and also the possible harmful effect due to toxic 

 excretions from the roots. 



2. When the food supply is limited the dominant factor of competi- 

 tion is that of food and in particular the amount of available nitrogen. 

 Other things being equal the total growth as measured by the dry 

 matter produced is determined by the nitrogen supply, irrespective of 

 the number of plants drawing on the resources. 



3. With limited food supply the efficiency index of dry weight 

 production decreases with the number of plants, as the working capacity 

 of the plant is limited by the quantity of material available for building 

 up the tissues. 



Ann. Biol, vi 12 



