II DEVELOPMENT AND ENVIRONMENT 35 



sown, as in the field crop, the uppermost laterals from 

 adjacent plants begin to encroach on each other's territory, 

 within three months from the date of sowing ; although 

 interlacing takes place, the form of the effective root- system 

 is modified by this lateral limitation into a cylindrical 

 upper portion ending below in a cone. The depth of the 

 cylindrical portion increases as lower and lower laterals 

 come progressively into contact. If the downward ex- 

 tension of the tap-root is checked, the conical portion is 

 overgrown, and the whole root-system becomes a cylinder. 

 The interest of this "root-interference" will become 

 apparent later, when we find that plants in field crop 

 behave as if they were " pot-bound." 



That the total volume of soil occupied by an adult 

 cotton root must be enormous is shown by the examin- 

 ation of pot plants. The photograph of Fig. 36 represents 

 an average plant from sowings in six-litre pots after 

 fourteen days ; the tap-root had attained a length of 

 16 cm. and was diverted by the floor of the pot after seven 

 days only. Having been exposed to higher temperatures 

 during the day than it would have experienced in the 

 soil, the absolute size of the root is somewhat abnormally 

 large, but the ratio of increase is interesting. After 

 seven days the total root-length was 20 cm. ; after fourteen 

 days, two metres ; after three weeks, four and a half 

 metres. Yet the growth-curves from the plants during 

 this last week showed that their root system was in- 

 sufficient for the needs of the stem, though the total leaf 

 area amounted to less than 30 sq. cm. 



On account of the more obvious importance of the 

 root system at a later stage, we shall at present omit 

 further discussion of its functions, but one experimental 

 result might be mentioned. With plants growing in pots 

 where the water content of the soil can be determined 

 with certainty, we find no obvious relation between this 

 factor of the environment and the growth of the stem 



D 2 



