CHAP, iv The Ash of Plants 363 



parts of the plant, which goes on regularly. It, however, 

 materially reduces the extent of such development, the 

 plants remaining extremely small. Some light was thrown 

 on the importance of this metal for the development of 

 the plant by Schimper's researches in 1890. Starting with 

 a water culture of Tradescantia in which only a minimal 

 quantity of potassium salts was supplied, he found that 

 as it became exhausted the new growth was checked. As 

 it proceeded the older leaves were gradually depleted of 

 the metal ; the new leaves formed were smaller and 

 thinner and finally became very minute. As the supply 

 from the older parts slowly failed, the tissues died, and 

 ultimately the growing-point itself perished. 



Macallum's observations, made quite at the end of the 

 century, brought further facts to light. He noticed that 

 the walls of the fibro-vascular tissue are saturated with 

 potassium, and that it is present in particular regions of 

 the protoplasm of the green cells, but never in the nucleus. 

 In the cells of the filament of Spirogyra he found a con- 

 centration of the metal in an area of the protoplasm near 

 and almost surrounding the chromatophore. In the higher 

 green plants he found a similar disposition of it near the 

 chloroplasts, but it was not so striking as in Spirogyra. 

 From his observations Macallum concluded that potassium 

 is associated especially with photosynthetic processes and 

 possibly also with those of dissimilation, and that it is in 

 some way concerned in the work of the chloroplasts. The 

 prominence of the metal in the structure of the cell 

 walls of the fibro-vascular tissue led him to the view that 

 it is concerned in the growth and formation of the cell 

 wall generally. Tracing the stages of the germination of 

 the spores of Equisetum arvense he found that the cell from 

 which the primary rhizoid arises is from the first very rich 

 in potassium salts, and that these collect at the point from 

 which the rhizoid subsequently springs ; as the latter con- 



