148 



X. THE CONIFEROUS STEM. 



callose, The sections need remain only a few minutes in the 

 stain, and can then be transferred to glycerine. This latter 

 allows the colour to remain in the sieve-plates only, removing it 

 from all other parts of the section. The sieve-plates cannot now 

 be overlooked in microscopical examination. Their colour is a 

 beautiful blue, and so permanent that the preparations can be 

 preserved. We can distinguish the sieve-plates even in the 

 immediate neighbourhood of the cambium, and follow them into 

 the parts in which the sieve-tubes have become crushed, and the 

 sieve-plates have therefore lost their radial position. Before this, 

 however, the sieve-plates will have lost their callus-layer, and 

 with it their capacity for staining. The sieve-tubes have the 

 form of the cambium cells from which they have developed ; they 

 bear sieve-plates only on their radial walls, just as trachei'des have 

 bordered pits. They present the appearance of round or oval 

 spots, which are collected into an indefinite number of somewhat 

 angular, finely-punctate areas (Fig. 55). At 

 some distance from the cambium the sieve- 

 areas are covered by a homogeneous shining 

 substance, staining sky-blue in the aniline blue 

 the callus-plate. Further away still this 

 is again dissolved, the sieve-area is bare, and 

 no longer stains ; the sieve-tubes are now 

 functionless. It is not difficult to see that 

 the active sieve-tubes contain a thin cyto- 

 plasmic lining layer, but as a rule no nucleus, 

 "^ f r~m> *kis having disappeared during the develop- 

 IffilSft) ment of the tube. They contain also watery 

 albuminous solution, grains which colour yel- 



FIG. 55.Pinus syl- low with iodine, and probably are leucoplasts, 

 vestris. Parts of two - ... . . -, n i 



.sieve-tubes with sieve- and likewise grams and nocculent masses 



which give with iodine the port- wine red 

 amylo-dextrin reaction, and probably represent an intermediate 

 product in the transformation of starch into dextrin and maltose. 

 These contents point to the sieve-tubes playing a very important 

 part in the nutritive processes in the plant. 



The crystallogenous cells of the bast are distinguished in 

 longitudinal section by their brown contents ; they are compara- 

 tively short, end commonly with horizontal walls, and have 

 apparently arisen from horizontal division of cambial cells. They 

 contain numerous prismatic crystals lying upon and near one 



