1995] CURRENT LITERATURE 389 
criticisms made on the cohesion theory of the ascent of sap by STEINBRINCK and 
by CoPELAND. 
STEINBRINCK established the fact that the walls of the conducting tracts are 
permeable to air and regards this as incompatible with the cohesion theory, since 
air diffusing through the tracheal walls would tend to break the continuity of the 
water columns within them, by the formation of free bubbles. In reply to this, 
Dixoy states that the air in solution does not cause the rupture of a water column 
under tension; that the permeability of the walls does not necessitate the forma- 
tion of free bubbles in the conducting tracts; and that even if bubbles are formed 
€ current is merely deflected from that portion of the channel. 
In regard to the results obtained by CopELAND, that the ascent of water in 
his “tree” was due not to a pull transmitted downward by the cohesion of 
water, but to some force which is measured by the difference in pressure as indi- 
cated by the manometers at the bottom and 8.4™ from the bottom of the “tree,” 
Dixon shows that according to CopELAND a column of water 8.4™ high, equiva- 
lent to 617™™ mercury, is supported by a pressure of 122™™ of mercury, which 
s impossible. Drxon further believes, and brings experimental evidence to 
Support his belief, that the pressure conditions indicated by CopPELAND’S manome- 
ters are local and have nothing to do with the true pressure conditions in the tube 
’sawhole. The arguments on this point are twofold. First, it is shown that 
plaster of Paris long continues to absorb water and this may cause the rise of 
mercury in the manometers; second, the rate of transmission of water through 
tubes of plaster of Paris as used by CoPELAND is so slow, that equalization of 
Pressure conditions by the passage of water through a distance of 8.4™ 1s 1mpos- 
sible—H. Hassetprine, 
Dixons describes an interesting transpiration model which will prove useful 
for illustration. The apparatus consists of a thistle tube closed by two parch- 
ment membranes so arranged that a lenticular cell is formed between them. 
Gelatin tannate is precipitated in the membranes, and the cell is filled with te 
“ugar. When the tube is filled with water the parchment cell becoming turg! 
Fepresents th . ich in living plants, intervenes between 
€ system of turgid cells which in living pt odel will act until a 
into the tube 
mig ‘o the fact that although the water in the cell may be 
ss oeived Substances exert a pressure. That this condi 
of plants is shown by the following interesting experiment. 
Bot. Soc. Trinity Coll. Dublin 
tion can exist in the 
A small strip 
we °*S Dixon, H. H., A transpiration model. Notes 
; 17-224, fig. F: 1905 H 
