542 THE STINGING PROPERTY OF THE GIANT NETTLE-TREE, 



distinct evidence of a concentration of acid fluid in the epidermal 

 cells, shown by the instant reddening of blue litmus solution, 

 while in the cells of the deeper tissues the indicator remains 

 unaltered for a considerable time. The extent to which the 

 above reaction will proceed, and therefore the quantity of free 

 acid formed (10) depends on the rate of removal of the active 

 mass of the products from the sphere of action. Now, one of 

 these is insoluble, it is found in the minute cystoliths in the 

 epidermal cells, and we shall next see how the free acids also are 

 removed, and thus the tendency of the reaction to proceed from 

 left to right is maintained. 



The sap is a very dilute solution of free acids and their salts. 

 Free acids, having a high diosmotic value, are removed from the 

 sap by diffusion through the primordial utricle of the cells, and 

 this seems to be the special function of the cells in the glandular 

 tissue of the basal cushions of the hairs. In these cells the 

 separation takes place, and the free concentrated acids are 

 secreted into the hairs. The acids are volatile, and constantly 

 evaporate from the open ends of the hairs, and so the second 

 product disappears from the scene of action, and the active mass 

 of the carbon dioxide from the air restores the equilibrium. 



We have said that the sap in the leaves of Laportea is a 

 0-224% solution of free acid, in which one gram molecular weight 

 of acetic acid is contained in 24 litres of water. Now, in this 

 state of dilution, acetic acid is dissociated to the extent of 2%, at 

 the ordinary temperature, and the free ions of both acetic and 

 formic acids are powerful plant poisons. Fassbender (11) states 

 that plants are killed by watering with a0'05 °/ solution of acetic 

 acid. It is here that we see the great significance of the neutral 

 acetate and formate, for these when present in the same solution 

 cause the degree of ionisation of the acids to be enormously 

 decreased, and so their activity as acid poisons is correspondingly 

 reduced. 



In the common nettle the free acid is probably all contained 

 in the hairs, with perhaps an exceedingly small quantity in the 

 sap. There is therefore no need for neutral formate in the 

 solution, and none is found. 



