108 PHARMACEUTICAL BOTANY 
fication of cellulose as to their walls. If transverse sections of 
either fresh or dried Buchu leaf-blades are first mounted in 
strong alcohol and observed under the microscope, the walls of 
the sub-epidermal cells appear no different from ordinary 
cellulose walls; if, however, the alcohol be gradually withdrawn 
from beneath the cover slip and gradually replaced with water, 
these walls will be seen to swell and form layers of mucilage in 
the cell lumina. The mucilage formed absorbs water so rapidly 
that, in a relatively short time, the pressure (turgor) within the 
cells becomes so great as to cause a rupture of the less elastic 
vertical walls of the sub-epidermis with a consequent separation 
of the upper epidermal cells from those of the palisade layer 
beneath and the appearance of a broad rent in the section. 
In pathological conditions, as a result of natural or induced 
injury to the tissues, the walls of cellulose and pectose may be 
completely transformed to mucilage. This condition is called 
gummosis. ‘The gum drugs Acacia and Tragacanth are produced 
in this way. 
The walls of some bacteria, other fungi and some algae are 
infiltrated with chitin. Those of the outer. layers of stems of 
grasses, sedges and horsetails contain silica deposits, while 
many plant walls contain calcium oxalate and calcium carbonate. 
GROWTH IN AREA AND THICKNESS.—The cell wall when first 
formed is limited in both extent and thickness. As the proto- 
plast within enlarges, new particles are placed within the wall 
by the process called intussusception. This increases its area. 
New particles, also, are deposited on its surface which gradually 
increases its thickness. The latter process is known as growth 
by apposition. 
Pores (Prrs).—Pores or pits are small thin spots or holes in 
the cell wall. The following terms are used to describe the 
character of these structures: 
Beaded. Circular pores arranged close together in a single 
row. 
Bordered. Pores partially covered on either side of the middle 
lamella by a thickened, overhanging projection of the cell wall. 
Branching. Pore canals which divide and sometimes sub- 
divide. 
