OSMOTIC PRESSURES IN THE POTATO PLANT AT VARIOUS STAGES OF GROWTH

Abstract

The first two observations on leaves of Ulmus are comparable since the leaves were situated under similar conditions; the last two records were obtained from leaves from the outside of the crown of the tree in one case and from a very sunny position in the last instance.Shade seems to lower the osmotic pressure even more than a location at the extreme tip of the tree increases it.The effect of shade on the osmotic strength of the cell sap was even more clearly brought out by a series of observations on the leaves of Syringa vulgaris: Covered 6 days, 1.263° depression, 15.20 atmospheres Exposed 6 days, 1.470° " 17.68 Covered 12 days, 1.010° " 12.15 Exposed 12 days, 1.608° " 19-34 Covered 21 days, .963°" 11.58 Exposed 21 days, 1.505° " 18.10Dixon says, regarding other observations on the sap from the roots as compared to that of the leaves: "In Syringa the pressure of the sap of exposed leaves was found to vary from 14 to 24 atmospheres, while that of the roots lay between 4 and 6 atmospheres.In Eucalyptus the osmotic pressure of the leaves ranged between 6.1 and 8.4 atmospheres; that of the roots was 5.3 atmospheres."The effect of wilting on the leaves of Syringa vulgaris is as follows:Control, 1-352° depression, 16.26 atmospheres Exposed to light without water supply for 4 hours, 2.002° " 24.07 " Exposed to light with water supply for four hours, 1.586° " 19.08 " Dixon states as one of his conclusions that: "Other things being equal, mature leaves showed a higher osmotic pressure than developing leaves."No detailed study could have been made at this time as the observations were confined to the months of September and October and the latter part of August.Dixon and Atkins (6), at a later time, observed the osmotic pressure in the leaves of Syringa vulgaris throughout an entire growing season and found that the rise was constant after April but that a rapid decline occurred a few weeks before the leaves dropped from the stem.In his book on "Transpiration and ascent of sap" Dixon (4) in a footnote says: "In almost every case it was found that the older leaves, caeteris paribus, had a higher osmotic pressure on the same plant.This was observed in Syringa vulgaris, Vitis Veitchi, Eucalyptus globulus, Hedera helix, and especially in Ilex aguifolium.The leaves of the last named evergreen persist through four or five periods of growth, and it is generally found at any time that the osmotic pressure of the sap of the leaves of each successive growth is lower than that of those which precede it."Hanning's observations (9) on the differences between the osmotic