OSMOTIC PRESSURES IN THE POTATO PLANT 
199 
2 A superior osmotic pressure is not necessary to maintain an organ 
after it has been formed. The leaves during July and August do not have 
as high an osmotic pressure as do the stalks, but they are able to maintain 
themselves and produce large quantities of starch. It seems necessary to 
assume, therefore, that the transpiring organs are connected directly with 
the root system by a long series of tubes in which the cross walls offer a very 
slight opposition to the hydrostatic head while their side walls are com- 
paratively impermeable. The rapid recovery of an herbaceous plant from 
wilting would support this theory, as would also the sudden drop in osmotic 
pressure of the sap from the leaves between ii A.M. July 29, and 10 A.M. 
July 30, during which period a heavy rain occurred. The plants on July 
29 were in a state of incipient wilting. The water that came to them must 
have passed through a long zone in the stalk where the osmotic pressure 
was greater outside the water-conducting tubes than it was inside them; 
still, these leaves did not wilt to any great extent. The question of the 
maintenance of turgor in the leaves of plants like the beet seems to have 
been overlooked by Dixon (4) in his theoretical discussions of sap flow, 
a though the conception he presents (pp. 141-142) of transpiring leaf cells 
at the upper end of a long tube (the tracheae) is the correct one. The 
osmotic pressure in these leaf cells is a measure of the pressure in the tracheae 
and in the conducting tissue, but not necessarily of that in the tissues in 
which the water tubes are imbedded. It would seem, therefore, that to 
the conception of transpiring cells at the upper end of a series of tubes 
should be added the idea of a direct connection of these tubes with the 
absorbing cells of the roots. Otherwise, the beet root would pull all the 
water from the leaves. The pressure in the garden beet itself, according 
to the determination on September 28, was 11. 81 atmospheres while in the 
leaves it was only 8.83. These pressures are comparable to those obtained 
by Pringsheim (14, page 135) for beets growing in damp soil, namely 11.68 
atmospheres. The leaves were found by the same investigator to maintain 
a pressure of 15.52 atmospheres while still young, but with the growth of 
the leaf the pressure fell to 5.30 atmospheres, at which point it remained 
constant. The observations recorded in table i on October i for sugar 
beets and sugar beet leaves are even more divergent in their differences. 
The beets had a pressure of 17.18 atmospheres, while in one case the outer 
leaves had 9.65 and the inner, 11.42 atmospheres. All the leaves taken 
together from another beet plant gave 12.21 atmospheres. The soil water 
in the tracheae supplying the leaves from the fine rootlets must be practi- 
cally impermeable to the high osmotic pressures surrounding them. Some 
water might be lost inward from one leaf cell to the adjacent ones nearer 
the stalk in the potato plant when the stalk has such a preponderance in 
osmotic pressure over that of the leaf, this process adding to the loss out- 
ward from transpiration. The total amount withdrawn by the cells may 
be small, but it may be the small excess that is necessary for the preservation 
