SIEVE-TUBES 113 



SIEVE-TUBES 

 i. Gucurbita 



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 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 it will be seen 

 on examination under a low power that 



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

 walled band of sclerenchyma with lignified walls (yellow, with 

 chlor-zinc-iodine, or acidulated aniline sulphate). 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 



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