THE CELL-WALL.—CELLULOSE. 23 
elaborated molecules of cellulose (C,H,,0-.), passes them to its 
outer surface, where they form a thin, colourless, transparent, 
continuous membrane. This membrane increases in thickness 
by the intussusception of new molecules between the older ones, 
and eventually there are generally developed upon it various 
markings, which may either be protuberances as in the case of 
some pollen-cells (fig. 73), and frequently of the cells forming the 
hairs on the surface of plants ; or internal depressions, as may be 
seen in spiral, annular, reticulated, scalariform, and pitted cells 
(see pp. 42-46). Those cells which are isolated, or on the surface 
of the plant, have the various markings on their outer or free 
surface, while those that are united to form tissues have them on 
Bac. oc. 
Fig. 37. Acell from the root of the Lizard Orchis (Orchis hircina). a. The 
cell-wall. 6. The protoplasm contracted by alcohol, ec. The nucleus with 
anucleolus. After Thome. Fig. 38. Cell with nucleus and nucleolus and 
vacuoles. Fig. 39. Cells from the root of Fritillavia imperialis. h. Cell- 
wall. &’. Nucleus. #, %. Nucleus with nucleoli. p. Primordial utricle. 
p', p’. Protoplasmic threads. s, s. Cell-sap cavity. s/, s’. Vacuoles. After 
Sachs. 
the internal surface of their cell-wall. The former is termed 
centrifugal thickening ; the latter centripetal thickening. 
This cellulose is insoluble both in cold and in boiling water, 
also in alcohol, ether, and dilute acids; but entirely soluble in 
an ammoniacal solution of oxide of copper. By the action of 
strong sulphuric acid at ordinary temperatures, the cellulose is 
disintegrated and converted into dextrin, and then, if water be 
added and the mixture boiled, the dextrin is converted into 
