ROTATION OF CROPS 193 



out, but, at all events, there would appear 

 to be little doubt that some organism be it 

 a fungus, a bacterium or a stem eel-worm 

 increases abundantly so long as red clover 

 plants are within its reach, but which can 

 be starved out by taking steps to avoid giving 

 it access to its particular host-plant. Simi- 

 larly in the case of the cabbage, turnip or 

 swede crops, which, as is only too well known, 

 are preyed upon by a minute fungus that 

 induces cancerous swellings and subsequent 

 decomposition in the roots. This fungus 

 can live only upon cruciferous plants, and 

 although, even in the absence of the plants 

 on which it preys, it can retain its vitality 

 for some years, it is starved out and de- 

 stroyed if the interval between the crops 

 that it affects is made sufficiently long. 

 Under some circumstances an interval of 

 four years will suffice, but under others a 

 longer period than this must intervene 

 between the growth of two cruciferous crops. 

 In this connection it may also be mentioned 

 that the continuous growth, or the growth 

 at too frequent intervals, of any particular 

 crop may tend to exhaust the land of some 

 necessary element of plant food, and this cause 

 also supplies a reason for cultivating crops 

 under a suitable rotation. 



