60 PRACTICAL BOTANY. 



where two sieve-tubes are contiguous. The sieve-tubes 

 will be more easily recognised in sections which have 

 been stained with Eosin (see p. 12). 



For more accurate study of these structures, see sieve- 

 tubes of Cucurbita (p. 84). 



5. 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 cellulose walls 

 (pitted, but not very distinctly) and protoplasmic contents. 



B. The Cambium, a band (here very narrow) of 

 oblong cells with very thin walls, and dense proto- 

 plasmic 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. 



C. The Xylem, consisting of 



a. Vessels, which are its most prominent constituent. 

 They are structures with lignified walls (note reactions), 

 which are variously marked ; they have no protoplasmic 

 contents, their wide cavity containing water or air. The 

 cavity is continuous, owing to the partial or complete 

 absorption of the transverse or oblique septa. Note 

 instances of this partial or complete absorption. 

 According to the various markings, or thickenings, of 

 their walls, the vessels may be grouped under the 

 following heads, the first named being the nearest to 

 the periphery of the stem : 



