1915] BROW N—PINUS STROBUS 205 
line between the compressed sieve tubes and those which exhibit 
no compression. Further, as will be shown subsequently, the 
latter are the first sieve tubes to function the following spring. 
We may say that in white pine phloem development continues 
longer than xylem development. It only ceases with the extreme 
cold temperatures of December and January, and the tree makes 
no special provision for cessation of growth as in the xylem. Sieve 
tubes in all stages of arrested development may be found during 
the dormant period. 
General discussion of tree growth 
Growth as it occurs in trees falls logically into two subdivisions: 
growth in length and growth in thickness. In the first category we 
have only primary growth. It does not matter whether elongation 
is going on in root, stem, or leaf, it always has its inception in a 
growing point, and all tissues resulting from cell divisions in this 
apical meristem are primary tissues. Growth in thickness, on the 
other hand, results mainly from secondary thickening which is 
brought about through the activities of a perennial cambium. 
Tissues arising in this way are distinguished as secondary tissues in 
contrast to primary tissues. | 
The primary tissues, with the exception of the primary cortex, 
usually soon attain their full size in both coniferous and dicoty- 
ledonous trees, and in the majority of woody plants we may 
regard them, with the one exception mentioned, as mature at 
the end of the first growing season.® Secondary growth, however, 
commonly begins the first year, and as a result the processes of 
primary and secondary thickening overlap, and both often go on 
at the same time in closely neighboring parts of the tree. Second- 
ary thickening may thus occasion alterations before all the primary 
tissues have reached the adult state. It is entirely conceivable, 
for example, that both categories of growth go on simultaneously 
in the terminal shoot of a pine or in a young root. In this connec- 
tion URsPRUNG (35) reports in certain cases the subsequent enlarge- 
ment of the pith after secondary thickening had begun, so without 
° It is only in woody monocotyledons and tree ferns that primary growth persists 
for any length of time. 
