O. N. Purvis 339 



The resistance of the plant to disease is very dependent on the supply 

 of potassium salts; fungoid and other pests find potash-starved plants 

 an easy prey, while neighbouring plants with a better potassium supply 

 remain much more healthy, though exposed to the same infection. 



In addition to these effects, there are others which, though equally 

 well marked, have received less attention from investigators. One of the 

 most striking is the effect on the strength of the stem. Though growth 

 is promoted in a wet season, there is considerable weakening of the stem, 

 and, in heavy winds, a consequent tendency for the plant to fall down 

 imder its own weight. The application of manures, especially of those 

 rich in nitrogen, increases this tendency, and since harvesting of 

 "lodged" crops is extremely difficult, the amount of growth-producing 

 manures which can safely be applied is limited by the ability of the 

 straw to stand uj) against rain and wind. It has, however, been demon- 

 strated in the field experiments at Rothamsted that an application of 

 potassium salts strengthens the straw in some way, so that larger quan- 

 tities of growth-producing fertilisers can be applied. This strengthening 

 has been observed elsewhere. R. P. Wright (3) in I89G found that an 

 application of kainit (MgS04 . KCl . SHjO) which contains 12-5 per cent. 

 KjO, increased the sohdity rather than the length of straw in oats. 

 Lodging occurred where sodium nitrate was applied alone, but where 

 kainit was also added there was no lodging, and the crop stood well. 

 Close, White and Ballard (4) .state that whereas carnations produce weak 

 stems with nitrogenous fertilisers, they were strengthened by phosphates, 

 and to a smaller extent by potassium salts. This strengthening is also 

 the general experience of farmers who use potassic fertilisers. 



This might be due to strengthening of the mechanical tissues, or to 

 some other effect, such as an influence on the chemical composition of 

 the walls or on the physiological condition of the plant. 



Few attempts have been made to account for this effect of potassium 

 salts, and they have led to no conclusive results. 



Oopeland{5) found that potassium salts increased the turgor of root 

 cells to a greater extent than do sodium salts, and that they are stored in 

 considerable quantities in the cell sap. If this were the case throughout 

 the plant the increased turgor would considerably enhance the standing 

 power of the plant. This question of turgor has not been fully investi- 

 gated and liiay prove of great importance in relation to the subject 

 under consideration. 



Kissel (C) found that potash had no definite effect on the stems of 

 oats, but in the case of grasses, especially of Avena elatior, the presence 



