in DEVELOPMENT AND ENVIRONMENT 79 



that the plant becomes more and more dependent, for 

 longer and longer periods, upon deeper and deeper layers 

 of soil for its steady water-supply. 



If now we decrease the depth of available soil by rais- 

 ing the water-table, and at the same time asphyxiate, or 

 ultimately kill, the lower part of the root-system, we are 

 throwing more strain on the surface roots, and reducing 

 the deep-seated reserve. If this reduction is effected 

 in July or August, when the amplitude of displacement 

 of the functional C.G. is at its maximum, the effects — 

 shedding, for instance — will be much more severe than if 

 the reduction is effected at a later date, when the leaf- 

 surface has increased but little more, when the surface 

 climate is damper, and the sun-temperature is lower ; all 

 these alterations tend to relieve the water-strain on the 

 roots, by reducing the evaporation from the stem. 



In this interpretation we find a reasonable explanation 

 for the otherwise disproportionate severity of the effects 

 produced by an early Nile flood, shown in the shedding- 

 curve and flowering-curve on the terraces in 1909, and — 

 according to the author's interpretation and forecast ^^' ^'^' ^^ 

 — by the whole of Egypt in the same year, with disastrous 

 results. 



Root regeneration. — The appearance of a root-system 

 in December is most remarkable, if it has been partially 

 immersed in the sub-soil water. The fullest examination 

 which the author has made was effected on the Gezira at 

 Cairo, where the sub-soil water-level is controlled entirely 

 by the river. By excavating a trench of three metres' 

 depth near the side of the plants, washing away the soil 

 with a jet of water from a force-pump, and using the 

 Ancient Egyptian method of reflecting mirrors to illuminate 

 the deeper portions, it was shown that the tap-roots had 

 descended to a depth of not less than 220 cm. Below the 

 depth of 160 to 170 cm., however, ail the original tap-roots 

 and their branches were dead (Figs. 35, with 37 and 38). 



