O. N. Purvis 349 



increase was seen which, however, was too small to be considered 

 significant. There was no significant difference between the plants from 

 the two plots in the thickness of xylem walls. 



B. In xylem. 

 Plot 8. Receiving minerals without potassium salts. 



At first the mean lumina were practically equal, but afterwards, 

 from May 31st the diameters were considerably higher where potassium 

 salts had been withheld, except on June 15th. The sudden drop between 

 June 8th and June 15th may be due to extreme pressure exerted by 

 rapidly expanding cells of the ground tissue (Fig. 10). 



Fig. 11 shows that the ratios of lumen to wall were equal at the 

 beginning of the investigation, but on May 31st and on June 8th, they 

 were higher where no potassium salts had been applied. On June 15th 

 the ratio in Plot 8 material had fallen considerably below that in plants 

 which had received potash but by June 26th had risen slightly above it. 



In the xylem the addition of potassium salts had no effect on the 

 wall thickness, but reduced the diameter of the lumen. This is the 

 reverse of the result obtained from sclerenchyma, but it must be remem- 

 bered that the two types of cell are not comparable. The xylem attained 

 a permanent and fixed condition before the earliest material was col- 

 lected, and changes during the period of observation may be attributed 

 to purely mechanical forces acting on the vessels. In the sclerenchyma 

 the weakening effect of potash was most marked in the early stages, 

 and, had it been possible to collect material later in the season this 

 discrepancy might have been found greatly reduced. 



Effect of Potassium Salts when Nitrogen Fertilisers are 



ADDED. 



The effect on crops of the addition of nitrogen fertilisers is to increase 

 the luxuriance of the vegetation, frequently at the expense of mechanical 

 strength. Microscopic investigation has revealed the fact that nitrogen 



