151 



roots, and the irregular direction of the elements of the 

 wood. 



The wood consists chiefly of vessels with bordered pits, tra- 

 cheidsand wood-parenchyma. Stereom (libriform cells, etc.) is not 

 especially developed, nor are the elements which occur in the 

 wood particularly thick-walled. There is no difference between 

 the spring and summer (or autumn) wood, and the limits be- 

 tween the different an- 

 nual rings are therefore 

 not distinct. The ves- 

 sels in each annual ring 

 are usually localized in 

 a certain zone. In a 

 measure, this causes 

 the annual rings to be 

 differentiated, but as 

 already mentioned their 

 limits are not sharply 

 defined. 



The slight develop- 

 ment of stereom re- 

 sults naturally from the 

 oecological form of the 

 plant. Sometimes sev- 

 eral, sometimes a few, 

 broad primary medul- 

 lary rays pass through 

 the wood ; there are only 

 slight indications of secondary medullary rays; radial cell-rows 

 occur, which are belter defined than the other cells, but the 

 cells have by no means distinctly the character of medullary rays. ' 



The pith consists of thick-walled cells, which form trabe- 



Fig. 7. Diapensia lapponica. 



Transverse section of stem; highly mag. 

 (Julianehaab.) 



^ Grevel denies the occurrence of secondary medullary rays. 



