Meristematic Tissues and Protein Iso-electric Points 189 
such an argument. The idea that a plant meristem requires to be 
relatively free from water in order to persist in its synthetic meta¬ 
bolism has already been employed in reference to the apical meristem 
(Priestley and Tupper-Carey (8)) and here also it is now suggested 
that ultimately the growing protoplasm will be found to maintain a 
certain equilibrium reaction favouring the withdrawal of water by 
the osmotic action of the vacuolated cells bordering upon the 
meristem. 
A gelatin gel at its iso-electric point still retains a considerable 
amount of water by a mechanism which is far from clear and which 
may still be vaguely expressed under the term imbibition. Many 
proteins, however, precipitate at their iso-electric points and can be 
removed from solution as amorphous solids, retaining no more water 
than any other wet precipitate and drying as readily. The natural 
assumption at the present time appears to be that the normal meri¬ 
stem with protein at its iso-electric point has little or no more affinity 
for water than such a precipitated protein and as fast as water is 
released in the system, by the condensation processes accompanying 
synthesis, it is lost to the surrounding tissues. The cambial meristem 
should therefore remain under conditions permitting synthesis and 
growth, so long as the hydrogen-ion gradient is maintained and the 
necessary food materials are supplied. 
The Iso-electric Points of Plant Proteins 
An essential condition for the production of meristematic activity 
on the above hypothesis is that the most important proteins in 
tissues capable of growth should have iso-electric points at reactions 
lying between pH 3 and 6. Few published data exist as yet upon 
this subject, but a variety of tissues and tissue proteins (from ten 
diverse genera) have been examined in this laboratory. The un¬ 
published results justify the statement that in the cases investigated 
the iso-electric points of the principal proteins in plant tissues lie 
between the required limits, and are frequently about pH 4-4. There 
are slight indications that the proteins of the tissues of aerial organs 
may differ in this respect from those found in subterranean organs. 
Conclusion 
While the ground traversed in this brief statement must be 
covered again, probably many times and over a much wider field, 
before the generalisation suggested can be accepted even as a working 
hypothesis in physiological anatomy, it is hoped that a case has 
