II DEVELOPMENT AND ENVIRONMENT 33 



to which an unbroken tap-root has actually been followed 

 is two metres and twenty centimetres. This depth had 

 been attained between March 28 th and September 1st, in 

 soil where the sub-soil water level is four metres below the 

 surface until September (Fig. 30). 



The growth of the tap-root is arrested or diverted 

 horizontally upon arrival at saturated soil, and plants 

 grown in land with a constant high sub-soil water level 

 consequently possess insignificant tap-roots,* with a greater 

 development of laterals. 



In case of injury to the growing point of the tap-root 

 one or more of the lateral roots nearest the tip turns 

 downward and replaces it. 



The resistance offered to root-growth by soils of normal 

 texture appears to be negligible, both from the indirect 

 evidence of growth-rates, and from actual observation in 

 glass-sided boxes. 



The secondary roots normally begin to develop when the 

 tap-root has attained a length of some 12 cm. Their 

 original diameter is about half that of the tap-root, and 

 roots of higher orders arise from them. Their rate of growth 

 under the same environmental conditions is slower than 

 that of their tap-root; this phenomenon is one of those 

 commonplaces of observation which have never received a 

 satisfactory explanation. In spite of their slower growth, 

 they produce an enormous increase in root area, on account 

 of their numbers, and if the soil is carefully washed away 

 from a cluster of cotton seedlings about six weeks after 

 sowing (Fig. 35), the root system appears as a tangled 

 white gossamer web. Only a few of the rays of this web 

 survive (Fig. 37). 



Before discussing the factors controlling root growth, we 

 may advert to the general form of the root-system, which, 

 beginning as a vertical line, rapidly becomes an inverted 



* Audebeau. 



