Grossenbacher Radial Growtli in Trees. 5 



THE BEGINNING AND DURATION OP RADIAL GROWTH. 



Observations and statements regarding the commencement of 

 cambial activity or radial growth in spring are many but no 

 positive conclusion can as yet be drawn as to just where on any 

 particular species of tree it will begin one season after another. 

 In fact it seems to differ considerably for individuals of the same 

 species. 



According to Strasburger 8 in pines as well as in Picea, as 

 many as five layers of tracheids had been formed from the cam- 

 bium in one-year shoots and considerable elongation growth had 

 occurred by the first of May, while at the bases and on the 

 trunks of eight-year old branches the cambium was still inactive. 

 In case of Robinia Pseudacacia and some other species, how- 

 ever, radial growth was found to begin first on the trunk. But 

 in general cambial activity is said to begin in one-year shoots 

 just back of the unfolding buds and to proceed downward to the 

 larger branches and trunks on which it usually begins uniform- 

 ly and at about the same time from top to bottom. He found 

 that the cambium gives rise chiefly or almost exclusive to wood 

 cells 9 in spring, and as the vegetative season advances, the pro- 

 duction of phloem increases while that of wood cells decreases. 

 In trees of our zone wood formation is said to cease by mid-Au- 

 gust while that of the phloem continues practically up to the 

 end of the vegetative season. Wood cells are therefore usually 

 matured before winter but phloem cells sometimes enter the 

 dormant season in an immature condition. 



Pfeffer 10 also says that "the secondary growth of xylem in 

 trees begins and ends sooner than that of the phloem." 



Hartig 11 states that although no growth had occurred on April 

 15 on any of the sixteen-year-old trees under observation, by 

 May 5 the new radial growth in oak was about equal on all parts 

 of the trunk but that none had occurred underground ; while 

 in maple, though the buds were farther advanced than in oak, 

 the growth as yet was confined chiefly to the one-year shoots. 



8 Strasburger, E. Ueber den Bau und die Verrichtungen der Leitung- 

 sbahnen in den Pflanzen. Histologische Beitrage 3:494. 1891. 

 1. c. p. 282. 



10 Pfeffer-Ewart. The Physiology of Plants. 2nd revised Ed. 2:207. 

 1903. 



11 Hartig, Th. Beitrage zur physiologischen Porst-Botanik. Aligein. 

 Forst-u. Jagd-Zeit. 1857: 281-96. 1857. 



