Ill DEVELOPMENT AND ENVIRONMENT 75 



large cultivated field, with their rims just buried beneath 

 the surface ; cultivation could then be normal, and the 

 water-level in each tank could be exactly controlled by 

 pumps and tube wells. 



The amount of space which we have devoted to the 

 subject of shedding may appear disproportionate, but the 

 Egyptian reader will realise its relation to recent problems 

 concerning deterioration of yield. 



\ The centre of gravity of the root system. — A 

 metaphorical term is often of use in discussion, and the 

 words " centre of gravity " are here used in a metaphorical 

 sense. 



The C.G. of the root system may be considered in two* 

 respects ; either structurally, or functionally. The structural 

 C.G. is first situated in the radicle of the germinating 

 seed, from which it moves steadily down the tap-root so 

 long as the latter continues to grow. Unequal development 

 of laterals would displace the C.G. from the axis of the 

 root-system, but we shall assume that root-development is 

 perfectly symmetrical in its radial distribution. The 

 position of this structural C.G. is the resultant of the 

 length of the tap-root with the length, number, and 

 position of the lateral roots, A plant growing in shallow 

 soil, whose tap-root has consequently been arrested, and 

 whose laterals have developed uniformly over all parts of 

 the stumpy axis, will thus have a structural C.G. situated 

 mid-way between the surface of the soil and the limiting 

 lower stratum ; this stratum may be a rock or a water- 

 table. If this limiting stratum is removed, the tap-root 

 will descend further, and the C.G. will be lowered. 



The mere structural C.G. is of less importance than the 

 functional C.G., which must obviously change its position 

 from day to day, even when the structural C.G. is 

 constant. The magnitude of these changes depends on 

 various factors, most of which we have already discussed, 

 and although much tedious analysis of soil samples will be 



