168 PLANT ANATOMY, 
the latter, such as concentrated glycerin, alcohol, and fatty or 
volatile oils. The mucilage then appears contracted to a mass 
which no longer completely fills the cell, as, for instance, in 
Bulbus Scille. Occasionally the masses of mucilage show a 
stratification, which is rendered more prominent upon the addi- 
tion of alcohol, as may be seen in Radix Althee. In such cases 
it is to be assumed that a gradual, even though but partial, con- 
version of the cell-wall into mucilage has taken place, especially 
when the mucilage, as in salep, is colored blue by iodine and 
sulphuric acid, or is even soluble in ammoniacal oxide of copper, 
like pure cellulose. The latter is the case with the terminal 
Fig. 79.—Transverse section through tragacanth, in which may still be seen the rem- 
nants of the cell-membrane which has been converted into gum, and also isolated starch 
granules. 
member of the cellulose series, lichenin (see pages 123 and 
170), which is related to the varieties of mucilage. That the 
cell-walls are capable of passing entirely into mucilage has al- 
ready been noted (page 164). In the formation of tragacanth, 
not only the cell-membranes participate, but also the starch 
granules which were previously deposited in the tissue, and 
_ which to a slight extent are still retained as such in the fraga- 
canth (Fig. 79). Other constituents of the cells also occasionally 
take part in the formation of gum and mucilage. 
