226 HOW CROPS GROW. 
regular empty cells, the walls of which are, for the most 
part, externally united and appear as one, a. At the points 
indicated by 8, cavitics between the cells are seen, called 
intercellular spaces. A slice across the potato-tuber, (see 
fig. 52, p. 277,) has a similar appearance, except that the 
cells are filled with starch, 
and it would be scarcely 
possible to dissect them 
apart; but when a pota- 
to is boiled, the starch- 
grains swell, and the cells, 
in consequence, separate 
from each other, a practi- 
cal result of which is to 
make the potato mealy. 
A thin slice of vegetable 
ivory (the seed of Phy- 
; telephas macrocarpa), 
under the microscope, dry or moistened with water, pre- 
sents no trace of cell-structure, the cells being united as 
one; however, upon soaking in sulphuric acid, the mass 
softens and swells, and the individual cells are at once 
revealed, their surfaces separating in six-sided outlines. 
Form of Cells.—In the soft, succulent parts of plants, 
the cells lie loosely together, often with considerable inter- 
cellular spaces, and have mostly a rounded outline. In 
denser tissues, the cells are crowded together in the least 
possible space, and hence often appear six-sided when seen 
in cross-section, or twelve-sided if viewed entire. A piece 
of honey-comb is an excellent illustration of the appear- 
ance of many forms of vegetable cell-tissue. 
The pulp of an orange is the most evident example of 
cell-tissue. The individual cells of the ripe orange may 
be easily separated from each other, as they are one-fourth 
of an inch or more in length. Being mature and incapa- 
ble of further growth, they possess neither protoplasm nor 
Fig. 30.” 
