68 PRACTICAL BOTANY 



i. Examine first the sclerenchyma. This appears 

 as a half-moon-shaped mass of tissue consisting of ele- 

 ments with rounded cavity, in which may be recognized 

 the remnants of protoplasmic contents. The walls are 

 thick, and lignified (yellow with acidulated aniline 

 sulphate, or with chlor-zinc-iodine, see p. 38). They 

 also show differentiation into layers, of which the most 

 prominent is the bright-looking middle lamella. Per- 

 pendicular to the internal surface of the walls may be 

 seen pits. 



ii. The soft bast, or phloem, 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, chlor-zinc- 

 iodine). 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. (See the descrip- 

 tion of sieve-tubes in Cucurbtta) 



5. 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, but they are not 

 readily distinguished. 



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 



