STRUCTURE OF SIEVE-TUBES. 129 



lying for a fairly long time in this latter, the cell-walls are more 

 or less completely decolorised, while the contents of the sieve- 

 tubes retain the stain. Almost all the sieve-plated are placed 

 horizontally; only a few have an inclined position. Most of 

 them appear covered with a highly refractive callose substance, 

 and show, proportionally to this, a not inconsiderable thickness 

 (Fig. 47, C), so that they are evident with a low power. In our 

 aniline -blue preparations these callose sieve-plates are coloured 

 clear blue. In the interior of the sieve-tubes is visible a con- 

 tracted, sac-like, axial string (u\ This is a string of protoplasmic 

 slime or mucus, which broadens at its ends, and almost com- 

 pletely covers the sieve-plate. It is stained indigo blue. One 

 of the ends adjoining the sieve-plates is usually more densely 

 full of contents, which form a terminal "slime-plug". Besides 

 the axial sac, the sieve-tube shows, with very careful examination, 

 a thin lining layer of protoplasm, extremely thin, and adhering 

 in most places closely to the wall of the sieve-tube. A nucleus 

 is not present. In somewhat younger sieve-tubes the slime- 

 string can be often seen to protrude through the pores of the 

 sieve-plate from one constituent cell. of the sieve-tube towards 

 the other in the form of bladder-like or worm-like prolongations. 

 In older sieve-plates such prolongations can no longer be seen ; 

 the callose substance has augmented, and contracted the sieve- 

 pores, but through these contracted pores the slimy contents of 

 one constituent cell of a sieve-tube are continuous with those of 

 another (as in B). At the outer and inner edges of the vascular 

 bundle the sieve-plates, covered with a callus layer, are very 

 noticeable (Fig. 47, C). The callus-plates are clearly distinguish- 

 able from their high refraction, and are stained sky-blue. In the 

 mid-thickness of the callus-plate the sieve-plate is more or less 

 clearly recognisable. The callus-plate consists, therefore, of two 

 halves, belonging to adjoining cells of the sieve-tube ; fine pores, 

 which ultimately are only recognisable as delicate striae, traverse 

 the callus-plate and the pores of the sieve-tube. Where two 

 sieve-tubes are in lateral contact, small sieve-plates are formed 

 on the common side-walls. These also later on possess a uni- 

 lateral (c*) or bilateral callus-plate, and are thus very visible. 

 By the side of the sieve-tubes are the companion-cells (s) ; they 

 are much shorter, so that four or five correspond with the length 

 of a segment of the sieve-tube. The study of their development 

 shows that a companion -cell arises originally as sister-cell to a 



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