84 PRACTICAL BOTANY. 



SIEVE-TUBES. 

 i. Cucurbita. 



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 found in herbaceous stems. 



I. Cut transverse sections of the stem of Vegetable 

 Marrow, stain with eosin, and mount in water or glyce- 

 rine. 



The general arrangement of tissues in this stem differs in 

 several important points from that in the Sunflower, and, indeed, 

 from that in most herbaceous Dicotyledons. Thus : 



1. There occurs at a short distance below the epidermis a thick- 

 walled band of sclerenchyma with lignified walls (yellow, with 

 Schulze's solution, or aniline sulphate and H 2 S0 4 ). This is quite 

 distinct from the vascular bundles. 



2. The vascular bundles are always separate and distinct : 

 though an interfascicular cambium is formed in old stems, no 

 secondary vascular tissue is derived from it. 



3. The structure of the individual bundle is abnormal, there being 

 in each bundle a central mass of xylem with the phloem masses 

 lying, the one on the central, the other on the peripheral side of 

 it. Between the xylem and the peripheral phloem mass is the 

 cambium layer. The structure is the same in both phloem masses : 

 either will therefore serve for the study of the sieve-tubes. 



In the soft bast, which resembles that of Helianthus, 

 but has larger constituents, observe 



(i). The transverse, circular, punctate Sieve-plates, 

 having the same appearance as in Helianthus, 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 longitudinal wall. 



