BAST MEDULLARY RAYS. 149 



another. The starch-containing cells likewise stand out clearly. 

 They are still shorter than the crystallogenous cells, lie in rows 

 one above the other, and are also interpolated singly or in longer 

 rows between the crystallogenous cells. Later, these starch- 

 containing cells swell very considerably. 



A similar section of fresh material, treated with concentrated 

 solution of molybdate of ammonia in saturated ammonium chloride 

 solution, shows a tannin precipitate in the crystallogenous cells. 

 Similarly if a piece of stem be placed for two or three days in 

 a concentrated aqueous solution of potassium bichromate, and 

 sections afterwards taken, a brown precipitate is found in the 

 cells. 



The medullary rays can be easily traced from the wood into 

 the bast, and in doing so undergo a characteristic change. In 

 order readily to appreciate this we lay some of our radial longitu- 

 dinal sections in iodine glycerine, and examine them in it. Those 

 cell-rows which in the xylem are trachei'dally developed, can be 

 for the most part traced into the bast as protein-containing cells. 

 These albuminpus cell-rows of the medullary rays are dis- 

 tinguished by greater height and smaller length, and are closely 

 applied to the sieve-tubes, with which they are connected by 

 sieve-plates. In that portion of the bast in which the sieve-tubes 

 have become functionless, these albuminous medullary-ray cells 

 have also lost their contents, and become crushed. From this we 

 infer that these protein-containing medullary-ray cells have in the 

 Scotch fir the function of companion-cells. These albuminous 

 medullary-ray cells contain no starch, while the starch-containing, 

 and also some of the trachei'dal, medullary-ray cells of the wood 

 can be traced into starch-containing rows of medullary-ray 

 cells of the bast. Protein-containing cell-rows in the medullary 

 rays are not found in the bast of all the Coniferae ; in their place 

 protein-containing rows of cells may be present in the bast 

 parenchyma. We may also note that in the Scotch fir these 

 albuminous medullary-ray cells first appear in the secondary 

 growth, and are not found in the primary bundles. 



The tangential longitudinal sections, which we likewise pre- 

 pare from alcohol material, must be taken through at least two 

 positions, namely in the wood and in the bast. 



The wood section shows us the trachei'des with unilaterally 

 tapering ends. The bordered pits which have been cut through 



