114 PEACTICAL BOTANY 



being in each bundle a central mass of xylem with the phloem 

 masses lying the one on the central, the other on the peripheral 

 side of it : this type of bundle is termed bi-COllateral. Between 

 the xylem and the peripheral phloem mass is the cambium 

 layer. The structure is fundamentally the same in both phloem 

 masses : either will therefore serve for the study of the sieve- 

 tubes. 



In the soft bast, which resembles that of Helianfhus 

 but has larger constituents, observe 



(i.) The transverse, circular, punctate sieve-plates, 

 having the same appearance as in Helianthus, and 

 easily recognized by their contents being stained with 

 eosin. 



(ii.) The companion-cells, appearing as though cut 

 off from the side of a sieve-tube by a curved wall. 



(iii.) Cambiform cells. 



Treat some sections with chlor-zinc-iodine : all the 

 walls of the soft bast turn blue (cellulose), but the 

 sieve-plates appear yellow or brown. 



II. Cut longitudinal sections through the soft bast : 

 either radial or tangential sections will do. Mount 

 some in iodine solution. The transverse sieve-plates 

 will be brought into prominence by the deep yellow- 

 ish brown staining of the mass of substance, which 

 surrounds them : this may consist of 



a. A callus-mass, which surrounds, and often com- 

 pletely invests the sieve-plate : the size of the callus- 

 mass is variable according to season, age, &c., being 

 greatest in autumn, and in old sieve-tubes. 



b. Protoplasm, which is usually collected in close 

 contact with the sieve-plate (or with the callus if 

 present), and more especially on its upper side, 



