60 



STRUCTURAL BOTANY 



to the surface of the stem at a point opposite the 

 bundle. As already mentioned, it is characteristic of 

 the class to which the Wallflower belongs to have this 

 actively dividing layer of cambium between the xylem 

 and phloem. Vascular bundles which have a cambium 



FIG. 23. Sieve-tubes and companion-cells from the phloem 

 of the Pumpkin (Cucurbita). A, Transverse section, 

 showing sieve-plate and companion-cell, z. B, Longi- 

 tudinal section, showing a sieve-plate in sectional view : 

 pr, primordial utricle of sieve-tube ; w, its contracted 

 contents; z, companion-cells. (7, Longitudinal section, 

 showing sieve-plates covered by callus c and Cj. D, Con- 

 tents of sieve-tube after the cell- wall has been dissolved 

 away with sulphuric acid. (After Strasburger. ) Magnified 

 about 400. 



are known as open bundles, because there is no 

 definite limit to their growth in thickness, the forma- 

 tion of new xylem and phloem from the permanently 

 active cambium going on as long as the plant lives. 

 We shall, however, understand the nature of tha 



