igiSl BROWN—PINUS STROBUS 2ll 



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 accompUshed 

 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 Hgnified. 



Evidence of the rapidity of xylem formation is readily obtain- 

 able in early May. It is not uncommon to find 10-15 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, 

 (&) 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. Gorp (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 



