Seasonal Variations in the Proportions of Suberized and Unsuberized Roots of Trees in Relation to the Absorption of Water

Growing root tips usually constituted less than 1 per cent and mycorrhizal roots less than 6 per cent of the total root surface under a 34‐year‐old pine stand. Growing root tips usually constituted less than 1 per cent of the total root surface under a yellow poplar stand, although one sample taken in May contained 9 per cent of unsuberized roots. The water permeability of various types of roots was measured under a pressure gradient of 31 cm of mercury. It differed widely among individual roots, ranging from an average of 6.6. mm 3 /cm 2 /hr for suberized pine roots 1.33 mm in diameter, to 36.6 mm 3 for suberized pine roots 3 mm in diameter, and 178 mm 3 / cm 2 /hr for unsuberized roots grown in water culture. Water intake through a group of unsuberized roots grown in soil averaged 37.4 mm 3 /cm 2 /hr. The permeability of yellow poplar roots varied even more, ranging from essentially zero to 30,000 mm 3 /cm 2 /hr. It is concluded that the major part of water absorption in pine occurs through suberized roots, some through mycorrhizal roots, and relatively little through growing root tips. Likewise, in yellow‐poplar most of the water probably enters through suberized roots. Further study is needed of the role of suberized roots in water and salt absorption.