THE VASCULAR TJSSUES. 



59 



Fig. 40.— a, Two sieve-tubes in transverse section, showing sieve-platea, fi, 

 and companion-cells, c (Cucurbita). b. Portions of two adjacent sieve- 

 tubes seen in longitudinal section. Note the sieve-plate, its perfora- 

 tions being plugged by oallose. The granular mass on the upper 

 surface of the sieve-plate is the callus. The cytoplasm is contracted 

 towards the centre of the tubes. c. Portions of two sieve-tubes 

 showing a pervious sieve-plate. 



