AMERICAN ' ,mn m 
JOURNAL OF BOTAIsr%^JJr^ 
Vol. VI May, 1919 No. 5 
OSMOTIC PRESSURES IN THE POTATO PLANT AT VARIOUS 
STAGES OF GROWTH 
B. F. LUTMAN 
The study of concentration of cell sap in plants has followed, in general, 
two paths: (i) the relation of the osmotic strength to the physical environ- 
ment of the plant, and (2) the part which the varying pressures in the roots, 
stems, and leaves play in the rise of sap. 
Since the publication of the paper by Drabble and Drabble (7) on the 
relation of the osmotic strength of cell sap in plants to their surrounding 
conditions, other authors have studied the same problem, especially under 
the extremes to which desert vegetation is subject, since it is in desert plants 
that the pressure is at a maximum. 
The other phase of this study has dealt with the relation of osmotic 
pressure to sap flow. Dixon (4, 5, 6), either alone or in collaboration with 
other investigators, has made the most extensive contributions to the solu- 
tion of this side of the problem, although the work of Hanning (9) should 
also be mentioned. Dixon and Atkins' (5) observations are so closely 
related to those made in this paper that they require more than a passing 
mention. 
The osmotic pressure should be higher in the leaves at the top of a tree 
than in those at its base, if Dixon's theory of the upward pull in the columns 
of sap in the water-conducting tissue is correct. His observations in general 
confirm this hypothesis, although there are occasional discrepancies due, 
as he believes, to the resistance in the conducting tracts. One of his most 
consistent series of observations, secured with Wistaria sinensis leaves, 
furnishes the following data: 
Shaded leaves from 3 feet level, 0.412° depression, 4.95 atmospheres 
Exposed leaves from 3 feet level, 0.437° " 5.25 " 
Exposed leaves from 27 feet level, 0.550° " 6.61 " 
Similar records secured with Ulmus campestris show: 
Leaves from short shoots at 18 feet level, 0.888° depression, 10.68 atmospheres 
Leaves from short shoots at i foot level, 0.763 " 9.18 " 
Leaves from short shoots at outer end of 
arched branch in shade at 10 feet 
level, 1.030° " 12.39 ** 
Leaves from short shoots on trunk in 
sunny position at 10 feet level, i.550° " 18.64 " 
■ [The Journal for April (6: 1 31-180) was issued May i, 19 19.] 
181 
