i66 THE POTASSIC FERTILISERS [chap 



facture of carbohydrates, like starch, sugar, and cellulose, 

 is greatly reduced, and in practice it is the crops rich in 

 carbohydrate which are most dependent upon a full 

 supply of potash. Reed observed that starch grains 

 were not formed in the cells of a green alga immersed 

 in a culture fluid containing no potash, and that 

 those originally present in the chlorophyll gradually 

 disintegrated and disappeared. Some earlier experi- 

 ments of Hellriegel and Wilfarth illustrate the depend- 

 ence of starch formation on potash even more clearly. 

 They started a series of water cultures, in one of which 

 the supply of nitrogen increased in successive jars from 

 nothing to the full amount required for the plant ; the 

 other constituents were fully supplied in every jar. In 

 a second series, the phosphoric acid supply was similarly 

 varied, and in a third series the potash. As would be 

 expected, in each series the amount of dry matter grown 

 was roughly proportional to the supply of the constituent 

 which was in defect. Further, when nitrogen or 

 phosphoric acid was lacking the formation of grain 

 was small, but, as far as might be, the grains produced 

 were perfect ; a larger number of grains, but not bigger 

 ones, being found as the supply of nitrogen or phosphoric 

 acid increased. Hence the weight of a single grain was 

 fairly constant whether there was much or little nitrogen 

 and phosphoric acid. But when potash was lacking the 

 individual grains were small and undeveloped, and the 

 average weight of each grain increased with each 

 addition of potash. In the absence of potash the 

 assimilation process was at a standstill, hence the 

 grains could not be filled with the starch which is 

 their main constituent. The following table (XLIII.) 

 shows the total dry matter, the percentage of grain, 

 and the weight of a single grain in each experiment 

 of the three series : — 



