390 BOTANICAL GAZETTE [November 



Microscopical characters of cambium and cortex in winter 



condition 



CROSS-SECTION 



The radial rows of tracheids in the xylem continue directly out 

 into the cortex (fig. 9) through the cambial zone. For a time this 

 radial arrangement is maintained, but sooner or later it becomes 

 irregular, due to certain changes which take place. The cambial 

 zone in cross-section appears as a number of layers of cells with 

 comparatively thin walls. It is impossible to pick out the initial 

 layer. Exterior to this are the sieve tubes. These have wider 

 lumina than the cells of the cambial zone, and the walls are thick- 

 ened as much as or more than those in the summer wood section 

 of the xylem. However, they are not lignified as are the latter. 

 Companion cells are wholly lacking. The rows of bast parenchyma 

 are very prominent. One row with a few scattered individuals is 

 formed each year (Strasburger 13), so that the thickened layers 

 of sieve tubes are separated by thin bands of bast parenchyma. In 

 the outer cortex the bast parenchyma cells become gorged with 

 starch and greatly enlarged. As a result the older sieve areas are 

 stretched tangentially and are seen as thin bands separating the 

 larger cells. Pith rays appear as straight lines running out into 

 the cortex, but as they proceed radially into the cortex they soon 

 become more or less irregular and curved. There are no crystalloge- 

 nous cells such as are described by Strasburger in Ptnus 

 sihestris. Exterior to the cortex proper there is a series of corky 

 layers which have arisen from living cells in the outer cortex, the 

 so-called cork cambiums. Their structure is of the general type 

 described by Strasburger (13). 



RADIAL 



In radial section the cambial cells appear as prisms with sloping 

 ends. The size varies slightly with the age. The sieve tubes have 

 the general shape of the cambium cells from which they originate. 

 Their radial walls are equipped with sieve plates, and these have 



same 



In radial section likewise we see to best advantage the bast paren- 

 chyma. This consists of rows of barrel-shaped cells arranged one 



