264 HISTOLOGY OF STEM OF THE WAX PLANT 



suberization was the result of interfiltration, it would seem as if it 

 should proceed /row the growing portion of the plant, and not toward 

 it from those that are dead. If the suberization was caused by the cell 

 walls absorbing the contents of the cell, it is but fair to suppose that 

 they would become suberized equally, although the opposite might 

 be the case. However it may be, in Hoya carnosa it would seem as 

 if some change which we call suberization took place in the cell wall, 

 caused by being cut off from the supply of nourishment. 



The Outer Band of Parenchyma 



This is most prominent in the younger parts of the stem and is grad- 

 ually reduced in thickness as the plant increases in age, till in the older 

 portions it is equaled in depth by the cork. It is composed of spheri- 

 cal cells having thin cellulose walls, varying in diameter from lo to 

 50 /x and increasing in size from the cork to the sclerotic girdle. The 

 cell walls dissolve readily in sulphuric acid and are turned blue by the 

 action of Schultze's solution. In the older portions of the stem they 

 become irregularly thickened and then present a pitted appearance. 



This band contains most of the chlorophyll and starch, also 

 protoplasm, sclerotic cells, milk tissues, crystals of oxalate of lime, 

 and the coloring matter of the younger portions of the stem. The 

 last is a rose-purple, apparently homogeneous fluid. Alcohol dis- 

 solves it rather slowly but much faster than it does the chlorophyll. 

 The addition of caustic potash changes the color to a sea-green, which 

 is quite distinct from the yellowish green of the chlorophyll. The 

 contents of all the cells of the bark are changed in color to an apple- 

 green, which decreases in intensity toward the cambium and deepens 

 in the vicinity of the cells that before contained the purple coloring 

 matter. Hence it would appear that this material is contained in all 

 the cells of the bark, but only in the outer ones does it become visible. 

 The green color produced by potash gradually fades and leaves the 

 chlorophyll granules apparently unaltered in color and form. Con- 

 centrated hydrochloric acid causes the color to change to a rich wine- 

 purple, which is confined to the cells containing the coloring matter 

 and does not spread through the other cells like the green caused by 

 potash. The color lasts perhaps ten minutes and then fades. Strong 

 sulphuric acid has the same effect, but the walls of the parenchyma 

 swell and are gradually destroyed. 



