THE MOLECULAR FORCES IN PLANTS. 241 



90. Corrosion Phenomena. 



Roots are not only able to supply plants with food staffs by 

 the absorption of ready-made food solutions, but they are also able 

 to withdraw from the compact elements of the soil absorbed, or 

 even still more firmly bound, substances. The absorbing cells of 

 the roots, especially the root- hairs, give out, at all events, as we 

 shall see in 91, certain substances which, on reaching the mem- 

 branes of the root-hairs, impregnated as these are with water and 

 closely applied to the particles of the soil, must exert a solvent 

 action on the soil particles. In this way the elements of the soil 

 undergo corrosion, and the substances thus dissolved by the agency 

 of the roots themselves pass 

 over into the plant. 



To prove that roots can set 

 up corrosive action, we make 

 the following experiment. A 

 small flower-pot is about half 

 filled with moist sand, on which 

 we now place a slab of marble 

 carefully polished on its upper 

 surface (the marble plate which 

 I used, and which in the course 

 of the experiment became 



corroded in the manner re- FlG - 89. Slab of marble, the surface of 

 , -i -.-,. orv . M which has been corroded by the roots of a 



presented in Fig. 89, was 45 Ph aseoius plant. 

 mm. in diameter and 7 mm. 



thick). The flower-pot is now completely filled up with moist 

 sand, and in this is laid a soaked seed of Phaseoius, which quickly 

 begins to germinate. The roots of the plant force their way 

 downwards into the sand, and after some time they encounter the 

 slab of marble. Over this they grow horizontally till they come 

 to the edge of it, when they again grow more or less vertically 

 downwards in the sand. If we stop the experiment, remove the 

 marble from the soil, wash it with water and dry with a soft 

 towel, we shall find on its upper surface an exact representation 

 of the roots which were applied to it. The polish has been 

 removed at the places of contact between marble and roots. I 

 -obtained, as i/idicated in Fig. 89, rather broad lines of corrosion, 

 -clearly owing to the corrosive action of the root-hairs springing 

 laterally from the roots. The main root growing downwards 

 P-r- R 



