New YorK AGRICULTURAL EXPERIMENT STATION. ot 
stages of a tree, and the presence or absence of a ground coy- 
ering determine the place and severity of the effect. 
Why roots may be especially susceptible Sorauer® says 
the reason that the roots of many trees are killed during more 
or less severe winters, while the aérial parts are often unin- 
jured, is because of the difference in the relative maturity 
of root and branch tissues at the arrival of winter. Cambial 
activity begins first at the distal ends of twigs and, as con- 
ditions become more favorable, progresses down to the stem 
and roots. On the other hand, growth and development gener- 
ally continues much later in the roots than in the branches. 
Von Mohl is said to have found that cherry twigs had begun 
development in April while the root-wood cells of the previous 
season had not yet finished their development. Owing to these 
differences in tissue development of root and branch, the 
former is made up mainly of large cells, characteristic of 
spring wood. That character of root-wood cells, in addition 
to their immaturity at the approach of winter, is said to make 
roots more susceptible to injury than other parts. The leaves 
of root-injured trees, though developing normally in spring, 
are found to sudenly turn yellow in summer; often followed 
later by the death of the tree. In case the injury is confined 
largely to the aérial parts, it seems probable that either the 
roots were protected in some way, during the excessive cold 
period, or the low temperature came after the root-wood had 
matured sufficiently to be resistant. On the other hand, early, 
short cold-snaps may injure young shoots but fail to penetrate 
deeply enough to affect the roots or crown. The publications 
by Taft, Green and Ballou, Waite, Eustace, Selby (Bulletin 
192) and Morse, reviewed above, are of this type. 
Suggestive experimental evidence.— The observations and in- 
terpretations by Waite and by Morse come nearer the experi- 
mental results obtained by P. Sorauer*! than the others, though 

* Pflanzenkrankheiten. 3rd ed. Vol. 1, pp. 561-64. 1909. 
“ Experimentelle Studien iiber die mechanischen Wirkungen des Frostes 
bei Obst- und Waldbiumen. Landw. Jahrb., 35: 469-525. 1906. 
