52 ti;a.\sactions and rr.ocEEmxGS of the [sess. lxi. 



examination of the crushed part of the stem shows that 

 while the vasa are collapsed, the medullary rays and false 

 rings are redm.-ed to mere white lines. The limit of eacli 

 annual rini; is marked by the close approximation of 

 several of thes.> •• false rings." 



Microscoi>l'- ilctnih (see plate p. 54). — In size, the vessels 

 are about the same as those of the palms and cucumbers. 

 In the spring wood, the radial diameter is 0".")() mm., while 

 the tangential iliameter is O'oO mm. Farther out, the vasa 

 diminish in size to 0"10 x 0'16 mm. 



The vasa occur singly, but pairs are pretty common — 

 the individuals of each pair are arranged radially with a 

 straight tangeiiual septum, so that each has a plano-convex 

 outline. 



Eadial triplets, however, occur, the external of the three 

 pores being usually small, and having the appearance of 

 having arisen by a tangential septum having been formed in 

 what was originally at the start a member of a twinned vessel. 



The tangential diameter of the vessels is, as a rule, some- 

 what in excess of the distance separating two adjacent 

 medullary rays, so that where they approximate a vessel 

 the rays are bulged outwards and separated from the vessel 

 by small-celled wood parenchyma. 



In some cases the parenchyma has grown into the vessel, 

 giving the well-known thyloses parenchyma common in 

 robinia, oak, etc. 



It is interesting also to note that in some vasa appear 

 the remains of what may be fungal mycelium. 



Mcdullarji I 'ays. — In an area of 2<y mm., one counts on 

 an average I •"> rays. 



The majority are multiseriate, but a few uniseriate rays 

 occur between the broader ones. The large rays are fairly 

 uniform in size, and run very continuously, diverging out- 

 wards where they touch a vessel. In many cases they 

 thicken on either side where they are crossed by or come 

 in contact with the false rings. Their breadth varies from 

 02 mm. to -0 7, and while the smallest consist of a single 

 row of cells, the largest are usually four cells broad. 



In their iiKiasurements the medullary rays approximate 

 those of some li-guniinous woods as given in N('»rdlinger's 

 Tables. 



