III.] ETCHING ACTION OF ROOTS 49 



the carmine and Indian ink show no signs of penetration, 

 because in their case the colouring matter is not dis- 

 solved, but consists of very fine solid particles suspended 

 in the fluid. Flowers, especially those from bulbous 

 plants, which readily absorb water, are occasionally dyed 

 by thus leaving them for some little time with their stems 

 dipping into a solution of some dyestuff which makes a 

 true solution. 



If the plant, then, can only take in materials that 

 have been first dissolved in the water in contact with the 

 root, how comes it that the plant can make any use 

 either of the soil or of a great number of manure 

 substances which are comparatively insoluble in water ? 

 This point will be more fully dealt with when we are 

 considering the soil, but at this stage we can show that 

 the plant itself helps towards bringing such substances 

 into solution. For the experiment, a thin slab of 

 polished marble will be wanted; the colour is of no 

 moment, provided that the surface is nicely smooth and 

 bright. This slab must be set vertically near the bottom 

 of a pot filled with ordinary soil in which two or three 

 dwarf beans are planted, preferably after germination, 

 in order to save time. When the beans have grown 

 pretty well and the pot is full of roots, shake out the 

 contents and wash the slab free from all dirt. The 

 polished surface will be found to be etched with a series 

 of markings representing the places where it has been 

 in contact with the roots, which thus evidently possess 

 some power of dissolving the carbonate of lime com- 

 posing the marble. From this experiment and the fact 

 that the sap contained in the roots of most plants is 

 acid to litmus paper, it has been somewhat hastily con- 

 cluded that the plant's roots excrete an acid sap, or 

 that the sap acts through the walls of the root and so 

 attacks the solid particles of the soil. Probably, however, 



D 



