OF THE EASTERN UNITED STATES 353 



The endodermis consists of tj'pical endodermal cells. It dies very 

 earlv and becomes filled up with a brownish substance which Chevaher 

 calls " ligneux-gommeuse " (gummy lignin). 



The pericambium is composed of one or two layers of large meriste- 

 matic cells which frequently give rise to side rootlets. The radial bun- 

 dle, according to Chevalier, (7, p. 97) possesses 4-8 tracheal poles, the 

 number varying in the same species. The writer has found the bundle 

 to be tetrarch in M. cerifera, M. Carolinensis, M. Macfarlanei and Com- 

 tonia asplenifolia. 



The pith region is composed of polyhedral cells with simple puncta- 

 tions on their transverse walls, and possessing small air spaces. Transi- 

 tion : — The external layer of the pericambium very early becomes a cork 

 cambium laying down cork on its outer face, thus cutting off the nutri- 

 ent supply and causing the tissues beyond e.g., epidermis, primary 

 cortex and endodermis to sluff off. On its inner face the cork cambium 

 cuts off secondary cortex. Secondary meristem develops on the inner 

 face of the phloem patches forming intrafascicular cambium as well as 

 on the outer face of the xylem strands forming there interfascicular 

 cambium and so a continuous cambium zone gradually arises which 

 cuts off secondary phloem on its outer face, thereby adding meta-phloem 

 and cuts off secondary xylem on its inner face thus augmenting the 

 total mass of xylem tissue. Secondary medullary rays are also cut off by 

 the cambium. The original radial structure of the stele changes to the 

 open collateral type. 



The primary phloem soon becomes inactive; the cells have their walls 

 thickened and frequently appear coUenchymatic. 



Secondary Structure 



Myrica cerifera, Linnaeus. (Plate 86. Fig. 16.) 

 In passing from periphery toward the centre the following pecuHari- 

 ties are to be observed : — 



1. Cork of several layers of irregular brick-shaped cells which vary 

 in their staining capacity. Many of the cells have highly suberized 

 walls which stain green with extract of chlorophyll. Some, however, 

 have all of their walls lignified, while others show lignification only on 

 their inner walls. The lignification of these was readily determined 

 through the use of phloroglucin and hydrochloric acid which imparted 

 the characteristic pink coloration. 



2. Phellogen of meristematic cells in a rapid state of division. 



