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 28th 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. 



