FIBRO VASCULAR TISSUES: PHLOEM 



ii 



simultaneously. A single row of parenchymatous elements is 

 represented as crossing the axis of the ray. To the left of the 

 cambium lies the wood with its tracheids and the continuation of 

 the ray. The radial structures in the xylem show also a differentia- 

 tion into central and marginal cells, but here ray-tracheids take 

 the place of the erect cells of the ray in the phloem. 



FIG. 88. Radial section through xylem, cambium, and phloem of the pine. 

 Explanation in the text. 



In Fig. 89 appears the tangential section of the tissues of the 

 phloem in Pinus. The plane of incidence of the section is tan-, 

 gentially slightly oblique; hence elements of different ages appear 

 on the opposite sides. Toward the right are represented sieve 

 tubes in which the protoplasmic lining has disappeared and the 

 sieve areas (seen here in profile) are blocked by masses of callus. 

 In the case of the rays which are naturally shown in transverse 

 section the marginal or erect elements are calloused on the side of 

 the tubes. The central cells of the rays still show the presence of 



