62 



CHANGES OF COMPOSITION 



[chap. 



Thus there was on the average a better crop by 30 per 

 cent, after the dry than after the wet winters, and this 

 may largely be set down to the restricted root develop- 

 ment in the wet seasons. Of course, a wet autumn in 

 other ways acts very prejudicially to the future crop of 

 wheat, partly by interfering with the sowing at the 

 proper time and partly by washing much of the soluble 

 plant food in the soil down below the reach of the 

 plant. 



When the winter is past and the wheat begins to 

 grow again, it enters upon its most active period of 

 drawing nutriment from the soil and of assimilating 

 carbon from the atmosphere ; the products, as fast as 

 they are manufactured in the leaf, are moved off and 

 stored up in the stems of the plants. As the lower 

 leaves age and begin to yellow off and die, the various 

 valuable materials they contain, not only the carbo- 

 hydrates that have been formed by assimilation, but the 

 nitrogen, phosphoric acid, and potash which are b^ing 

 employed in the vital processes, are very largely with- 

 drawn and passed on to some more active part of the 

 plant. These substances are of too great importance to 

 the plant to be wasted by being allowed to remain idle 

 in dead tissue or to be lost to the plant when the leaf 

 falls, consequently they are taken back into the active 

 parts of the plant as far as possible before the leaf dies. 

 Throughout the months of March to June, the processes 

 of manufacture and storage alone are going forward ; but 



