78 THE COTTON PLANT IN EGYPT CHAP. 



system, having dried up the soil particles near its root 

 hairs, has to wait for the incoming of water from adjacent 

 particles as reinforcements, so that the plant is injured 

 during the clay. The greater severity of the water-strain 

 during July, due partly to the higher evaporimeter read- 

 ings, and more to the greater leaf-area, is shown by the 

 greater water-loss during similar intervals, resulting in 

 more severe desiccation of the soil on July 30th as 

 compared with July llth. 



The water-loss is more severe in the upper layers of 

 soil, partly on account of surface evaporation, but more 

 from the complete occupation of such soil by the lateral 

 roots. The extent of this desiccation becomes more severe 

 as the plant becomes larger, and it extends to a greater 

 depth in the same time. In other words, the centre of 

 gravity of the absorbing root system shifts downwards 

 as the root grows larger, along with the structural C.G. 

 Moreover, even on the narrowest assumption, the top 40 

 cm. of soil could have been providing little or no water to 

 the roots it contained on July 30th, whereas it has been 

 making free provision on July 15th. Thus, on July 15th 

 the functional C.G. and the structural C.G. probably 

 coincided, whereas on July 30th the functional C.G. has 

 descended far below the structural. 



The extent of this displacement obviously increases as 

 the root grows larger, and as root interference between 

 adjacent plants extends to greater depths. Thus, in the 

 germinating seed there can be no displacement ; structure 

 and function have a common centre of gravity. In seed- 

 lings with a ten-centimetre root there is a slight change 

 as the surface soil dries up after watering, but the dis- 

 placement is slight. So, as we progress to older and 

 older plants, the amplitude of the displacement increases, 

 its duration being bounded by successive waterings, till we 

 reach a maximum amplitude probably in early August. 



Using a different method of expression, we may state 



