22 



MANUAL FOR SUGAR GROWERS. 



the root some acid exudation takes place, and this 

 attacks the plant-food, dissolving it and fitting it for 

 entrance into the root by the process of diffusion. 

 This power is well shown by an experiment devised 

 by Professor Sachs (" Text-book of Botany," first 

 English edition, p. 625). If polished plates of mar- 

 ble or phosphate of lime are covered with sand to a 

 depth of a few inches, and seeds are then sown in 

 the sand, the roots which strike downwards soon meet 

 the polished surface of the mineral and grow upon 

 and in close contact with it. After a few days an 

 impression of the root-system is found corroded in 

 rough lines on the smooth surface ; every root has 

 at the points of contact dissolved a small portion of 

 the mineral by means of the acid water which per- 

 meates its outer cell-walls. 



The author, therefore, in the process of soil-analy- 

 sis makes use of very dilute cold hydrochloric acid to 

 extract the available mineral plant-food, believing 

 that by this means he obtains a fair approximation 

 to the process of nature. 



The quantity of plant-food removed by a crop 

 varies, as has been said ; Warington gives the fol- 

 lowing quantities in pounds per acre. 



