70 PRACTICAL BOTANY 



SIEVE-TUBES 



i. Cucurbit a 



Though the sieve-tubes of the Sunflower are fairly large, the 

 soft bast does not occur in large masses. In the Vegetable 

 Marrow, however, the sieve-tubes are of extraordinary size, and 

 occur in large numbers : this stem is thus excellently fitted for 

 the study of the sieve-tubes of the type usually found in 

 herbaceous stems. 



The material should be hardened in alcohol. 



I. Cut transverse sections of the stem of the Vegetable 

 Marrow, stain with eosin, and mount in water or glycerine. 



The general arrangement of tissues in this stem differs in 

 several important points from that in the Sunflower: the 

 structure of the individual bundle is also abnormal, being of the 

 bi-collateral type. 



In the soft bast, which resembles that of Helianthtis but has 

 larger constituents, observe 



(i.) The transverse, circular, punctate sieve-plates, having 

 the same appearance as in Heltantkus, and easily recognised 

 by their contents being stained with eosin. 



(ii.) The companion-cells, appearing as though cut off from 

 the side of a sieve-tube by a curved wall. 



(iii.) Cambiform cells. 



Treat some sections with chlor-zinc-iodine : all the walls of 

 the soft bast turn blue (cellulose), but the sieve-plates appear 

 yellow or brown. 



II. Cut longitudinal sections through the soft bast : either 

 radial or tangential sections will do. Mount some in iodine 

 solution. The transverse sieve-plates will be brought into 

 prominence by the deep yellowish brown staining of the mass 

 of substance, which surrounds them : this may consist of 



