The effects of temperature, photoperiod and chilling on the leafing‐out of Norway spruce, Picea abies (L.) Karst. were studied. High temperature promotes breakage of post‐dormancy, long photoperiods having no such effect. Photoperiod and chilling cause the breakage of true dormancy. However, under field conditions, photoperiod will have no effect on leafing‐out date in the spring. By use of clonal material it was possible to show substantial genetic differences between individuals in response to temperature and photoperiod. When the effect of clones was accounted for, treatments could be compared more precisely. Differences between clones were apparent in heat‐sum required for leafing‐out, in rapidity of response to favorable post chilling conditions, and in chilling requirement This latter quantity was given a new definition, applicable when both chilling and post chilling temperatures are controlled and specified. This is that period beyond which a further 10 days of chilling accelerates leafing‐out by less than one day, i.e. the point at which the slope of the line relating days till leafing‐out to chilling period, is equal to minus 0.10. Differences in leafing‐out date were shown between provenances taken from throughout the range of Picea abies. These differences were related to latitude with provenances of high latitude leafing‐out first.

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PLANT PHYSIOLOGY periments in which we have confirmed the observation by Galston and Baker (2) that elongation in the dark of green pea stem segments is much more dependent upon sucrose than is the elongation of etiolated stem segments. Furthermore, since we have detected no promotion by cobalt of the growth of the green segments, the condition affected by cobalt apparently is not limiting to elongation. In one experiment, the elongation of such stem segments (originally 5.3 nun long) was L52 mm on 10 mg/l IAA, and 4.02 mm on IAA plus 2% sucrose. The elongation on either solution was not increased by inclusion of CoC12 in the concentration range from 4 to 20 x 10-5 M. SUMMARYSucrose in some experiments slightly promoted and in others slightly inhibited the elongation of sub-apical segments from etiolated pea stems. The sugar, however, markedly increased the fresh weights of the segments. Cobaltous salts, when added to test solutions containing only indole-3-acetic acid (IAA), slightly promoted elongation. When sugar and cobalt were added together to the IAA solution, great increases of elongation occurred. The evidence reveals a distinction between the roles of water uptake and cell wall increase in the enlargement of plant cells.It is suggested that sucrose increases cell volume and that cobalt promotes the ability of the cell walls to increase in surface area.

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