1915] BROWN—PINUS STROBUS 211 
the others until after the first of May, but growth must have been 
in evidence in the higher portions of the tree before that date. In 
the latitude of Dresden growth apparently starts fully as early as 
at Ithaca. 
Intensity of growth in aerial parts 
As already noted, growth continues some time before cell 
division occurs. It is first manifest through the enlargement of 
tissues already formed during the previous year. When cell 
division begins, it proceeds at first very rapidly, and in such a 
way that more elements are added to the inside of the cambium 
than to the outside. This was plainly observed in the sections and 
included in the data in table C. There 8-12 tracheids have been 
formed, as compared with 2 or 3 new’sieve tubes. The cells thrown 
off to the outside gradually become transformed into sieve tubes, 
or more rarely into phloem parenchyma cells. This is accomplished 
in the first case through a thickening of the cell wall and the forma- 
tion of lateral sieve plates. The phloem parenchyma cells thicken 
their walls very little at first, but enlarge for several seasons and 
eventually attain a much larger size than the sieve tubes. In the 
outer bark their walls are often strongly lignified. 
Evidence of the rapidity of xylem formation is readily obtain- 
able in early May. It is not uncommon to find 1ro-15 tracheids 
fully formed (fig. 5) without any indication of thickening of the 
wall. Subsequently the thickening begins, and before it has 
progressed to any extent lignification is evident in the cell walls, 
as brought out by the phloroglucin-HCl reaction. Wall thicken- 
ing and lignification never start, however, until tracheids have 
attained their maximum dimensions as seen in cross-section. 
The rapidity of vernal growth in white pine is apparently 
contingent on three factors: (a) the amount of reserve food material, 
(0) moisture, and (c) temperature. The first is always at its 
optimum in the spring, as the abundance of starch in the storage 
tissues testifies. Moisture likewise, at this time of the year, is 
available long before the buds begin to open. GorF (8) has pointed 
out the early resumption of growth in the roots of coniferous plants, 
and observations on white pine coincide with his results. The 
