ii DEVELOPMENT AND ENVIRONMENT 37 



has passed into soil at constant temperature, containing 

 perhaps 70 per cent, of the saturation quantity of water, 

 the growth rate can safely be predicted. We have already 

 examined the effect of temperature on root growth, and it 

 suffices to add that the mean growth rate of many roots 

 growing in soil in the laboratory works out at approxi- 

 mately 0'9 mm. per hour at 18 C., and 1'25 mm. per 

 hour at 22 C. When the seedlings under examination 

 are placed out of doors, the total growth of the roots in 

 twenty-four hours is considerably less than the calculated 

 amount (see also Fig. 35), and it seems probable, although 

 a complete experimental proof has not yet been obtained, 

 that this diminution is due to the " sunshine effect," 

 which disturbs the water-equilibrium of the root conjointly 

 with that of the stem. 



The rate of growth under field conditions is not easily 

 ascertained. Small plants can be grown in boxes or tubes 

 with glass sides, which are buried flush with the soil 

 surface, and pulled up as often as may be necessary for 

 examination of the root, which grows down the inclined 

 glass side. For checks on such plants, and for examina- 

 tion of all large plants, we have no method other than the 

 primitive and muddy one of excavation with a jet of water 

 and a trowel till the end of an unbroken root is reached. 

 Measurements made by these methods have given data of 

 which the following series is representative : 



From these it follows that the root-depth in well-drained 

 soil certainly reaches two metres by the end of August.* 



* The lower soil-temperature in the Northern Delta should diminish the 

 root extension considerably. The point needs investigation on account of 

 the intimate relation to drainage projects. 



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