SIZE OF WOOD-CELLS. 143 



an average appears to be established. This fact is illustrated 

 bv the following table, based on measurements of tracheids of 



kJ 



Pinus sylvestris. 1 



Medium width of the 

 tracheids. 

 .017 mm. 



.032 mm. 



From this table it is seen that the increase can be traced up to 

 the forty- fifth year, but that from that time on. the tracheids in 



*j */ 



one ring- have the same length as those in the next. Those in 



O ~ 



the forty-fifth annual ring have an average length of about five 

 times that of those in the first. In the wood of oak, the libri- 

 form cells exhibited the greatest difference in length. Thus 



ci? < * 



Sanio found that in a stem of Quercus pedunculata, with 130 

 rings, the medium length of these elements in the ring of the 

 first year was .42 mm., and in the three outer rings 1.22 mm. 

 Tracheids in the same rings measured, however, only .39 mm. 

 and .72 mm. respectively. With this increment in the length of 

 wood elements in successive rings, Habeiiandt associates a fact 

 noticed bv Alexander Braun ; 2 namelv, that the wood elements 



/ / - 1 



in some stems and branches stand not parallel with the axis, but 



1 Ueber die Grb'sse der Holzzellen bei der gemeinen Kiefer, Prings. Jahrb., 

 viii. 409. 



2 Ueber den schiefen Verlauf der Holzfaser, und die dadurch bedingte 

 Drehung der Baume, Berlin, 1854. 



It is proper to refer at this point to an instructive paper by Abromeit 

 upon the histology of the oaks, in which the most marked characters of the 

 North American species are fully treated (Pringsheim's Jahrb., 1884, p. 209). 

 According to Abromeit, the oaks can be plainly classified as follows : - 

 I. With wide well-marked medullary rays. 



A. The annual rings distinctly defined by the concentric, circles of the 

 larger ducts of the spring wood, and seen by the naked eye. The 

 smaller ducts are arranged in radial rows in the autumn wood. 

 a. With thin-walled ducts. 



a. The radial rows of small ducts touch each other tangentially : 

 Quercus lyrata, alba, Durandii, stellata, macrocarpa, "Wislizeni 

 Prinus, Gnrryana, bicolor (var. Michauxii). 



