264 



SCIENCE PROGRESS 



These are starch-formers and may contain grains which are 

 coloured wine-red, instead of blue, by iodine, owing to the 

 formation of dextrin by enzyme action. 



Within the protoplasmic lining of the active sieve-tube ele- 

 ments is a solution of albuminous substances and sugars, together 

 with minute granules of proteid nature. The fluid may be quite 

 thin and watery or, as in the case of Cucurbita, it may be 



Oa0^> 



'4^mm:^'> 



Fig. 3. — Young sieve-plate. 



Figs. 4, 5. — Diagrams to show formation of callus-tubes and slime-strings. 

 A, cellulose wall ; B, callus ; C, protoplasm ; D, slimy cell contents. 



Figs. 6, 7.^Diagrams to show mature sieve-plates in section and surface view. 

 (Figs. 3-7 are founded on Hill's paper on " Sieve-tubes of Angiosperms.") 



Fig. %\ — Diagrammatic representation of phloem elements. 



A, protoplasmic lining of sieve-tube ; B, slimy contents, contracted, as seen in spirit material ; 

 C, phloem parenchyma ; D, companion-cell ; E, sieve-plate ; F, wall of sieve-tube. 



slimy and gelatinous. In the latter case clots of jelly are 

 produced on treatment with alcohol, due to the coagulation 

 of the albuminous contents. The actual composition of the 

 sieve-tube contents varies greatly from time to time and is 

 dependent upon external conditions. When actively functioning 

 a transverse cut through the sieve-tubes causes a partial ex- 



