1919] BRIEFER ARTICLES 44 1 



xylem in one or two year old stems. The average depth of the latter 

 almost always equals and usually exceeds that of the former (fig. i), 

 indicating conclusively that there is no growth acceleration in the 

 convex arcs of the cambium which form the projecting wedges. As has 

 been indicated by the writer, the undulating outline of the first formed 

 secondary xylem is due to the stellate arrangement of the primary 

 elements, and consequently the stellate outhne of the first formed cam- 

 bium. However, this originally lobed cambium rapidly takes on a cir- 

 cular outline, owing to the slower growth of its convex projecting arcs, 

 except in stems which have a hereditary tendency for the formation of 

 pairs of approximated multiseriate rays. — I. W. Bailey, Bussey Institu- 

 tion, Jamaica Plain, Mass. 



IMPORTANCE OF EPIDER^LAL COVERINGS' 

 (with two figures) 



In making tests of the relative resistance of some herbaceous plants 

 to freezing, it was observed that inoculation from ice formed on the leaf 

 surface was a factor of great importance in determining the temperature 

 at which ice formation occurred in the leaf tissue. In testing cabbages 

 it was observed that the greatest undercooling of the tissue below its 

 freezing point occurred in those plants which had the greatest amount of 

 " bloom " on the leaf surface. Plants well covered by wax could be main- 

 tained for hours at a temperature 5°C. below their freezing point without 

 the formation of ice in the tissues. Similar conditions were found to 

 occur in the common Cineraria and other such plants which are densely 

 covered with a mat of epidermal hairs. This condition suggested that 

 inoculation of the undercooled leaf tissue by ice formed on the leaf 

 surface was an important factor in frost resistance. The object of this 

 study was to determine the amount of undercooling which can occur in 

 such tissues, and the importance of the epidermal coverings in preventing 

 surface inoculation of the undercooled tissues. 



The thermoelectric method was used to measure temperatures, 

 since this method allows one to determine the temperature inside rather 

 thin leaves. A copper-constantan couple of no. 40 B. and S. gauge which 

 had a thermal coefficient of 3 .33 milhvolts per degree Centigrade was 

 used. Using such a couple the delicacy of the potentiometer arrange- 

 ment determines the accuracy of the temperature measurement. Al- 

 though with the arrangement used much smaller changes could be 

 ' Published by permission of the Secretary of Agriculture. 



