146 Some Factor ni in Plant Comjjetition 



The average results for the ten pots with each rate of seeding are 

 set out in Table I and Fig. 1. The quantity of dry matter produced 

 per pot is practically identical in each case, as the variations fall within 

 the limits of experimental error. This indicates that the nutritive 

 functions are severely limited by some factor of competition which 

 cuts down the normal activities of the plants in proportion to the number 

 of individuals that are struggling for existence in a similar bulk of soil. 

 With 1, 2, or 3 plants per pot the amount of nitrogen taken up is identical, 

 and it is evident that the whole of the nitrogen available for use as food 

 was used by a single mustard plant under the conditions of experiment, 

 and where more plants were grown per pot the individuals were unable 

 to obtain as much as they could utilise. Probably therefore, the chief 

 competitive factor in this case was the quantity of available nitrogen, 

 as when once this was all absorbed the plants were unable to make 

 further use of other nutritive substances. With 4 or 5 plants per pot, 

 on the other hand, the whole of the available nitrogen was not taken 

 up, the quantity lacking being slightly outside the limits of experimental 

 error. At the same time a sharp drop occurred in the green weight, due 

 entirely to the presence of less water, the dry weights being almost 

 constant. In this case some other limiting factor, probably not nutritive, 

 was apparently beginning to come into action, but the available data 

 at this stage do not allow of any certainty as to its nature. With 4 or 

 5 seeds per pot the plants matured very much earlier, and by the time 

 of cutting a considerable amount of desiccation had already occurred, 

 which would account for the drop in water content but not for the 

 reduced intake of nitrogen, both in actual weight and in proportion to 

 the total dry matter. 



The amount of water supplied to the pots was the same in every case, 

 and the amount of dry matter formed per pot was constant, irrespective 

 of the rate of seeding. Nevertheless the plants made very different use 

 of the water, the thinly seeded plants retaining far more in their tissues, 

 so that the succulence as shown by per cent, dry matter in green weight 

 decreased steadily as the plants became more crowded, from 1 to 4 or 

 5 per pot, 4 and 5 per pot being alike in this respect. This is not merely 

 a question of the quantity of water available for use by individual plants, 

 as with constant supply and constant dry matter it would be theoretically 

 possible to attain a similar degree of succulence in each case, but the 

 interplay of the various factors of competition is such that the balance 

 of the physiological processes is altered as crowding occurs and the 

 response is partly in the direction of change of succulence. 



