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.

Foresters have often concentrated their attention on characteristics of trees which had an immediate practical application and have neglected fundamental features. Diameter growth is a good example. Usually it has been measured at one position on the bole at 5- or 10-year intervals. Such a procedure ignores the biological unit of growth which is the annual layer of xylem laid down on every part of the stem. Some characteristics of that growth layer are described in this paper.

Radial files of tracheids having changing orientations were traced through serial tangential sections to deduce events in fusiform cambial cells (FCCs) associated with spiral grain formation. Within an induction bridge of phloem and cambium sloping diagonally across a stem girdle in Pinus contorta Dougl. ssp. latifolia, repetitive, homologously sloping, pseudo-transverse anticlinal divisions in FCCs were a primary event contributing to FCC orientation changes. Imperfect periclinal divisions (IPDs) in FCCs also contributed. In Picea glauca (Moench.) Voss, IPDs and FCC elongation interacted to change tracheid orientation during natural spiral grain development. In both natural and induced reorientations, small numbers of adjoining FCCs changed their orientation abruptly while the greater population of FCCs changed orientation less rapidly. These sites of abrupt reorientation are termed "microdomains."not available

Nine seed lots of Pinus banksiana Lamb. and one of P. contorta var. latifolia Engelm. were grown for 115 days on five nitrogen levels and two photoperiods. The nitrogen levels were: zero N—no nitrogen, [Formula: see text], [Formula: see text], 1N—203 p.p.m., and 2N—406 p.p.m. The two photoperiods were a long-night photoperiod (10-hour day) and a broken-night photoperiod (9.5-hour day followed by 0.5-hour light break in the middle of the dark period). The plants, five per vessel, were analyzed for their response to treatments in terms of growth, development, and nitrogen content.(1) The breaking of the night doubled the height, total dry weight, and leaf weight and increased root weight and nitrogen absorption, but it lowered the number of fresh, active root tips, the proportion of the plant in root tissue, and the nitrogen concentration, inhibited production of secondary leaves, and delayed winter bud formation.(2) The increase in nitrogen level in the medium lowered the percentage of fresh, active root tips, the proportion of the plant in root tissue, and the production of winter buds; it increased the content and concentration of nitrogen in the plants. Up to a certain maximum (about [Formula: see text]), it increased the dry weight and height of the plants.(3) The interrelation of growth and developmental stimuli is discussed. It is suggested that growth is an important factor in controlling development.

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