SUNFLOWER. STEM. 55 



thick, and lignified (yellow with acidulated aniline 

 sulphate, or with Schulze's solution). They also 

 show differentiation into layers, of which the most 

 prominent is the bright-looking middle lamella. 

 Perpendicular to the internal surface of the walls may 

 be seen pits. 



(ii.) The soft bast consists of elements of very 

 different structure and function : these are : 



(a.) Sieve-tubes, which appear in transverse section 

 as the larger cavities of the soft bast : their walls are 

 rather thin and consist of cellulose (blue, Schulze's 

 solution). Occasionally these cavities will be found 

 traversed by transverse septa, having a punctate appear- 

 ance. These stain dark brown with iodine solution. 

 They are transverse sieve-plates. (Cf. below, 

 description of sieve-tubes in Cucurbita.) 



(6). Abutting directly on the sieve-tubes, and appear- 

 ing as though they had been cut off from the sieve- 

 tube by a longitudinal wall, may be seen smaller cells. 

 These are the companion cells. 



(c). The remaining elements resemble the sieve- 

 tubes in transverse section except in their smaller size, 

 and absence of sieve-plates. These are cambiform 

 cells, or phloem parenchyma. 



Passing inwards, the distinction of these several 

 constituents of the soft bast becomes more difficult, 

 while the walls are thinner, and the arrangement of 

 the elements is more regularly in radial rows, till, in 

 the band of thin-walled tissue which borders imme- 

 diately on the xylem, these characters become very 

 obvious. This band is 



B. The Cambium, or active formative layer. Its 



