1915] BROWN—PINUS STROBUS 225 
Changes in temperature are less effective there because the thick 
corky layers tend to equalize the conditions which prevail at different 
times during the growing season. Cool autumn nights, for example, 
would chill the cambium in the upper parts of the tree much earlier: 
than below. Temperature changes become operative first where 
the primary cortex still persists, that is, where the bark is yet 
smooth. This without doubt explains the disparity of growth as 
we find it in white pine. Growth is first retarded above, but may 
go on vigorously below for a much longer period. 
The exact time of growth cessation apparently varies widely 
in different species, in different localities, and in different sites. 
While wide variations occur, still certain generalizations apply. At 
the outset the term ‘“‘growth’’ is a misnomer. As already noted, 
phloem formation, at least in conifers, does not cease with xylem 
formation, but continues uninterruptedly until late in the fall. 
It is necessary, therefore, to discuss xylem and phloem separately 
in their relation to cessation of growth. 
A comparison of cuttings from tree I for the’years 1912 and 1913 
will give an idea of the seasonal termination of xylem formation. 
One discrepancy was noted at the start. In spite of the fact that 
the final cuttings in 1912 were a week later than those in the follow- 
ing year, growth was apparently more vigorous at the later date 
in 1912 in all four cuttings. This is to be explained in two ways. It 
was due either to seasonal variations or to the fact that the vigor 
of the tree had been materially lessened the second year through 
the many cuttings taken from it. An examination of the meteoro- 
logical data for the two seasons has added no convincing evidence, 
inasmuch as comparative figures of growth for the two years were 
not at hand for a sufficiently large number of individuals, and 
general assumptions were therefore out of the question. Possibly 
both factors were in force. 
To give the exact time or a definite place in the tree for the 
termination of xylem formation is quite impossible, as the data on 
tree I indicate (table F). In 1912 growth was still in evidence on 
September 26 in all four cuttings, as transitional forms of tracheids 
could be noted in every case (fig. 6). Growth, however, was going on 
at this date very sluggishly. Often only one flattened transitional 
