SUNFLOWER. STEM. 59 



tudinal bands of small, oblong, thin- walled cells 

 (epithelium). 



The Bundle-sheath may occasionally be recognised 

 as the layer of cells immediately outside the bundle. 

 Very commonly starch grains may be detected in its cells. 



4. The Vascular bundle. Supposing the section 

 to have been approximately median through the bundle, 

 the following components will be found to be included 

 in it : 



A. Phloem, which is made up of 



i. Hard Bast, Sclerenchyma, or bast fibres. These 

 appear in longitudinal section as long prosenchyma- 

 tous cells, occasionally divided by more or less oblique 

 septa. Walls thick, lignified (yellow with Schulze's 

 solution, or with acidulated aniline sulphate), and 

 pitted : remnants of the protoplasmic contents may be 

 found, especially if the stem, cut be not very old. 



ii. Soft bast, consisting of tissues with cellulose 

 walls (blue with Schulze's solution) and abundant pro- 

 toplasmic contents : its several constituents are 



a. Sieve-tubes, long tubular structures with thin 

 walls and transverse or oblique septa (sieve-plates), 

 the structure of which is the chief characteristic of the 

 sieve-tubes ; they are readily recognised in sections 

 treated with Schulze's solution (or iodine solution) by 

 the deep brown coloration of the protoplasm, which 

 is collected round the sieve-plates. 



Treat some sections with potash : the protoplasm, 

 and mass of callus surrounding the sieve-plates, swells, 

 and the perforated or sieve-like character of the 

 septum, which does not swell, is then easily recognised. 

 Sieve-plates occur occasionally on the lateral walls, 



