28 Wi^-onshi Academy of Sciences, Arts, and Letters. 



\\er, round to develop in the second and subsequent annual rings 

 by tin' elimination of most of the wood cells between adjoining 

 rays, l-'ames 48 has noted a similar compounding of the simple 

 IM\S of white oaks. Rubner found that the ray cells in the val- 

 ley wood are shorter than those in the ridge-wood. The wood 

 in valleys often showed no indication of rings because the cells 

 weiv frequently all of the summer-wood type with a reduced 

 radial diameter. In the deep valleys many rings were found to 

 converge into a homogeneous layer of small cells many of which 

 had brownish contents. In some cases as many as twenty-two 

 year's growth had occurred on the ridges while no growth re- 

 sulted in the valleys. In some such instances the cambium in 

 the valleys had become thick-walled and apparently lost its 

 power of growth and in others it had died and turned brown. 

 In the smaller valleys of trunks phloem production was found 

 excessive while on the ridges it was only slight. Rubner also de- 

 scribed instances in which no radial growth resulted on the lower 

 portion of tree-trunks during a number of years. He found 

 that long branches with sparse foliage have very irregularly dis- 

 tributed radial growth, often being wholly omitted in some por- 

 tions and present in others, although at times with imperfectly 

 differentiated cells. Similar irregularities were also noted by 

 Ursprung 49 in branches of teak wood from the tropics ; cross sec- 

 tions showed that in some growing seasons the cambium had 

 been active in only a part of the circumference. 



The work reviewed above shows that several types of excen- 

 tric radial growth, occur both in horizontal and upright struc- 

 tures and that some of them are apparently due to differences in 

 bark pressure and to an excentric distribution of the transpira- 

 iton current and metabolized food, while in others the cause of 

 the excentricity is not shown. For instance these authors have 

 not determined why radial growth should be distributed in scat- 

 tered patches on branches or tree-trunks which have an inade- 

 quate supply of food or why fluted trunks and buttressed stumps 

 should occur, although Detlefson made some interesting sugges- 

 tions regarding the latter. Rubner has shown that radial 

 growth is very slight in the valleys or grooves occurring in the 



8 Eames, A. J. On the origin of the broad ray in Quercus. Bot. Gaz. 

 49:16166. 1910. 



Ursprung, A. Zur Periodizitat des Dickenwachstums in den 

 Tropen. Bot. Zeit. 62: Abt. 1:189-210. 1904. 



