72 PRACTICAL BOTANY 



them : observe transitional forms. The cell-contents 

 resemble those of (2), but there is less chlorophyll. 



Note the resin-passages, the course of which is 

 directly longitudinal; they therefore appear as longi- 

 tudinal bands of small, oblong, thin-walled cells 

 (epithelium). 



The bundle-sheath may occasionally be recognized 

 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 : 



i. Hard bast, sclerenchyma, or bast-fibres which 

 appear in longitudinal section as long prosenchyma- 

 tous cells, occasionally divided by more or less oblique 

 septa. Walls thick, lignified (yellow with chlor-zinc- 

 iodine, 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. The soft bast, or true phloem, consisting of 

 tissues with cellulose walls (blue with chlor-zinc-iodine), 

 and abundant protoplasmic contents : its several con- 

 stituents are 



a. Sieve-tubes, long tubes with thin walls and 

 transverse or oblique septa (sieve-plates), the struc- 

 ture of -which is the chief characteristic of the sieve- 

 tubes; they are readily recognized in sections treated 

 with chlor-zinc-iodine (or iodine solution) by the deep 

 brown coloration of the protoplasm, which is collected 

 round the sieve-plates. 



