Walter Stiles 
j6 
8. Determination of Permeability and Absorption by Colori¬ 
metric Estimation of the Hydrogen Ion Concentration 
of the External Solution 
Changes in the concentration of hydrogen ions in the external 
solution can also be determined colorimetrically by the indicator 
method (see Clark, 1920). The scope of the method is exactly the 
same as the preceding, although the results obtainable are not so 
accurate. Using this method Haas (1916 b) has shown that no excre¬ 
tion of acid apart from carbonic acid takes place from the roots of 
maize and wheat dipping into distilled water. No excretion of alkali 
occurs in the case of maize, while with wheat there is only a very 
slight increase in the alkalinity of the water after the roots had 
decayed. 
9. Test of Permeability by Metabolic Action 
While ash analyses have shown what substances are normally 
absorbed by plants, growing plants on artificial media of known com¬ 
position has shown which elements are necessary for normal growth. 
If such an element is supplied to a plant in only one compound and 
the plant grows healthily so that its metabolism is normal, it follows 
that that substance must be able to penetrate into the plant. Thus 
it has been shown that although nitrogen is most suitably provided 
to most higher plants in the form of nitrates, in many cases other 
nitrogen compounds are readily absorbed. Thus ammonium salts, 
urea, various amino-acids such as glycine, asparagine, leucine and 
tyrosine, uric and hippuric acids, acetamide and propylamine are 
among nitrogen compounds that have been shown to be absorbed by 
various species by means of the continued growth of the plants when 
supplied with no other source of nitrogen. The literature dealing with 
the possible sources of nitrogen for Fungi and Bacteria is enormous 
and cannot be entered into here. 
The penetration of inorganic salts through the leaves of Thunbergia 
and other plants was shown in this way by supplying the roots with 
nothing but water and spraying the leaves with a solution of nutrient 
Salts, or by immersing them in the solution (Dandeno, 1901). 
The healthy growth of algae such as Nostoc (Bouilhac, 1897) and 
Stichococcus (Artari, 1901; Matruchot and Molliard, 1902) in the dark 
on solutions containing glucose or other organic sources of carbon 
indicates the permeability of the cell membranes to the sugar or 
other organic nutrient. 
