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.Gr. 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 16 ' 17> 18 

 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, all the original tap-roots 

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



