SUNFLOWER STEM 73 



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 sep- 

 tum, which does not swell, is then easily recognized. 

 Sieve-plates occur occasionally on the lateral walls, 

 where two sieve-tubes are contiguous. 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. 113). 



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 develop- 

 ment : these are, however, difficult to distinguish, but 

 their presence is proved by the transverse sections. 



< . Bast-parenchyma, or cambiform 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. 103, and Fig. 9, B, p. 107). 



iv. The xylem, consisting of 



a. Vessels, which are its most prominent constituent. 

 They are elements with lignified walls (note reactions 

 as on p. 38), which are variously marked : they have no 

 protoplasmic contents, their wide cavity containing 



