iQiSl BROWN— PINUS STROBUS 



20S 



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 (ss) 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. 



