526 BOTANICAL GAZETTE [DECEMBER 
year. Whether the differences shown by the bark and wood girdled material 
may be accounted for by a decreased moisture supply in the latter is an open 
question. It is interesting to note, however, that many plants grown with a 
deficiency of water do show an increased tannin content. 
e it is impossible to draw broad conclusions from the results pre- 
sented, the work constitutes a genuine contribution toward a more nearly 
complete knowledge of the causes of the responses following girdling, and adds 
to the available information on the entire problem of growth. In any future 
work it would be particularly desirable to follow the nitrogenous compounds 
and mineral nutrients as well as the carbohydrates, more especially with a 
view toward the determination of the ratios of these various substances in 
relation to the observed responses.—E. J. Kraus 
Imbibition—MacDovucaL® and MacDovucar and Spornr™ are doing 
work on the effects of acids and bases on imbibition of water by plant tissues 
and plant gels that promises to be the most significant contribution in this 
phase of plant physiology that has been made for some decades. Practically 
all of the work on the effect of acids and bases on the amount of swelling and 
force of swelling of gels and on the viscosity and osmotic pressure of sols has 
been done on the amphoteric protein gels. For these it seems well established 
that the iso-electric point (the reaction at which the particles are without a 
charge) is the point of minimum swelling, force of swelling, osmotic pressure, 
and viscosity, and that forcing the ionization of the gel or sol either to the posi- 
tive by addition of an acid or to the negative by the addition . a base increases 
the swelling, osmotic pressure, and viscosity very markedly. 
MacDovceat and Spornr find that both base set acid aii (n 0.01) 
decrease greatly the swelling of agar plates and to a less degree of Opuntia 
tissue. In fact, Opuntia tissue acts more like mixtures of gelatine and agar 
than it does like either gelatine or agar. These results suggest that in con- 
trast to the protein gels and sols, the point of maximum swelling, viscosity, etc., 
in agar is the iso-electric point and that the positive agar due to acid addition 
or the negative agar due to base addition shows a lowering of these characters. 
In this connection it is to be regretted that the H+ concentration for the iso- 
electric point of agar has not been determined. It is also desirable to know 
the behavior of various other carbohydrate gels and sols (mucilages, pectic 
materials, gums, etc.) to see whether this contrast in behavior is a general 
difference between the protein and carbohydrate gels. It seems that plant 
physiologists have generally assumed that the laws of behavior of protein 
% MacDoveat, D. T., Imbibitional swelling of plants and colloidal mixtures. 
Science 44:502-505. 1916. 
™ MacDoveat, D. T., and Spornr, H. A., The behavior of certain gels useful in 
the interpretation of the action of plants. Science 45:484-488. 1917 
*s Hoper, R., Physik. Chemie Zelle Gewebe. 329-338. 1014. 
