151 
roots, and the irregular direction of the elements of the 
wood. 
The wood consists chiefly of vessels with bordered pits, tra- 
cheids and 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 ineach annual ring 
are usually localized in 
a certain zone. Ina 
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- 
Fig. 7. Diapensia lapponicu. 
lary rayS pass through Transverse section of stem; highly mag. 
(Julianehaab.) 
the wood; there are only 
slight indications of secondary medullary rays; radial cell-rows 
occur, which are better 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- 
! GREVEL denies the occurrence of secondary medullary rays. 
