Meristematic Tissues and Protein I so-electric Points 187 
across a marked gradient of hydrogen-ion concentration, but that 
the gradients run in opposite directions and differ markedly in 
range. Any attempt to elucidate the significance of this fact necessi¬ 
tates some discussion of recent work upon the colloid chemistry of 
the proteins. 
The Iso-electric Point of a Protein 
Colloidal systems dispersed in water frequently bear an electric 
charge and, as a result, if an electric current is passed through the 
solution, the charged particles migrate slowly to the oppositely 
charged electrode, a phenomenon known as electro-cataphoresis (see 
Burton (4)). It has long been known that the electric charge upon 
the dispersed particles varies with the reaction of the dispersal 
medium, and also that there is one reaction of the medium at which 
the colloid will behave as if uncharged. This reaction Hardy (6) has 
termed the iso-electric point. Loeb(7) has developed further the 
significance of this fluctuating electric charge in certain types of 
colloids, and as a result of his work it is evident that for proteins, 
at least, the position of the iso-electric point is one of the most 
significant properties of Ihe pure substance. Proteins may behave 
either as acids or as bases, in virtue of the carboxyl (—COOH) and 
amino (NH 2 —) linkages of their constituent amino-acids. Loeb has 
shown that if a pure protein is dispersed in a medium containing 
electrolytes, which is more acid than its iso-electric point, then the 
protein bears a positive charge, behaves as a base and forms in part 
a protein-acid salt. On the other hand, in a medium more alkaline 
than its iso-electric point, the protein is negatively charged, behaves 
as an acid radicle and forms salts of the base-proteinate type. With 
this key to their behaviour, Loeb has shown convincingly that the 
complex molecules of the proteins combine according to the ordinary 
stoichiometric laws of classical chemistry. A very confused and 
perplexing field of colloid chemistry has thus been reduced to some¬ 
thing like order. At the moment, however, we are only concerned 
with one of the very wide range of phenomena that have been 
elucidated. At the iso-electric point some of the physical properties 
of a protein are at a maximum, others at a minimum. Thus at this 
point proteins show most tendency to precipitate and a minimal 
tendency to swell in water (Loeb (7) and literature cited therein). In 
other words if a protein gel or the solid phase of a protein sol be 
under consideration, with changing reaction of the medium there 
will be a change in the distribution of the water within the protein, 
