FROST RING FORMATION IN THE STEMS OF SOME CONIFEROUS SPECIES

Seedlings of several conifer species were artificially subjected to freezing temperatures. Microscopic examination of sections, taken at intervals after the frost, revealed the way in which frost rings developed. Differentiating tracheids and xylem mother cells were killed by the frost, leaving a permanent band of underlignified and crumpled tracheids inside a band of dead cell tissue. Most of the cambial initials remained alive but developed abnormally into short irregular tracheids. Parenchyma cells proliferated mainly from the xylem ray cells. With subsequent growth, the growing stresses, which had become subnormal because of the collapse of killed cells, were restored. This was accompanied by the reestablishment of the cambium to its normal form.