SUNFLOWER STEM 43 



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 constituents are 



a. Sieve-tubes, long tubes 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 recog- 

 nized 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. 



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 recognized. The sieve-tubes will be more easily 

 recognized in sections which have been stained with eosin 

 (see Appendix A) which stains the contents of the tubes 

 deeply. 



A more detailed study of sieve-tubes, and their structure 

 and contents will be given below in a special section (p. 70). 



b. Side by side with the sieve-tubes may be found the 

 companion-cells, which are smaller sister-cells of the segments 

 of the sieve-tubes, cut off during development. 



c. Bast-parenchyma, or cambifcrm cells. These are oblong 

 parenchymatous cells with thin, indistinctly pitted, cellulose 

 walls, and protoplasmic contents. 



iii. The cambium, a narrow band of oblong cells with very 

 thin walls, and dense protoplasmic contents. As the tissue in 

 this case differs in no essential point from that in other plants 

 treated elsewhere, and as it is here difficult to study, its 

 description will be deferred, though its presence here must not 

 be forgotten (see below, p. 64, Fig. 7, B). 



iv. The xylem, consisting of 



a. Vessels, which are its most prominent constituent. They 

 are elements with lignified \valls (note reactions as on p. 25), 

 which are variously marked : they have no protoplasmic con- 

 tents, their wide cavity containing during life water or gases. 

 The cavity is continuous owing to the partial or complete 



