270 ANATOMY OF STEMS. 



continuity of the medullary column is broken by lig- 

 neous plates, which proceeding from the side of the 

 central tube, either partially intersects it, or complete- 

 ly partition off portions of it, as in several of the Mag- 

 nolias ; while in others, again, it is merely a spongy 

 sheath, lining the interior of the cavity, as in the stem 

 and branches of Woodbine (LoniceraPericlymenwn). 

 Where the branches are given off from a stem, a 

 thread of medulla, in some instances, separates from 

 the central column, and entering the branch, is grad- 

 ually augmented to a diameter proportionate to that 

 of the branch. In the annual shoot, the wood shuts 

 up the canal of the pith at its extremity, as soon as it 

 ceases to grow for the season, as is seen in the longi- 

 tudinal section of the shoot of Horse Chesnut, imme- 

 diately under the terminal bud ; and thus isolates it 

 from the shoot of the next year. In many plants this 

 forms a kind of woody partition, which marks the lim- 

 it of the growth of each year in the length of the stem ; 

 but in others it is absorbed, the continuity of the pith 

 being, apparently, uninterrupted from the root to the 

 apex of such stems. Those partitions are almost al- 

 ways present when the pith is composed of distinct 

 plates, as in the Walnut, or of a spongy sheathing 

 membrane, as in Woodbine. 



The color of the pith, in the succulent shoot or in 

 the young plant, is green, which, as the cells empty, 

 changes to white ; but to this there are some excep- 

 tions. Thus it is yellow in the Barberry ; pale brown 

 in the Walnut; fawn-colored in the Sumach, (Rhus 

 Coriaria) ; and pale orange in the yellow flowered 

 Horse Chesnut ; but it is more frequently colored in 

 the caudex of the root than in the stem. 



A vertical or horizontal section of a thin slice of 

 pith, under the microscope, appears to consist of hex- 

 agonal cells, which are larger and more regular in the 



