42 STRUCTURE OF THE CELL-WALL. 
the Cryptogamous water plants, as Chara, the cells are also 
much elongated. 
III. GENERAL PROPERTIES AND STRUCTURE OF THE CELL- 
waLu.—As has been already stated (page 23), the cell-wall of 
young cells is very thin, colourless, transparent, smooth, and 
free from any openings or visible pores, so that each cell is a 
perfectly closed sac. The cell-wall, however, although free 
from visible pores, is readily permeable by fluids. 
As the cell-wall increases in age, however, it becomes thick- 
ened by the intussusception or incorporation of new matter into 
its substance, and then alterations occur 
by which it becomes variously marked 
and sculptured on its inner surface, 
This increase in thickness may be speci- 
ally observed in the prosenchymatous 
cells of the wood and inner bark, and 
in the hard cells of the stone of the 
Peach, Cherry, and other similar fruits. 
This thickening, however, of the cell- 
wallis by no means confined to the pro- 
Fig. 79. Transverse section of senchymatous cells of the wood, or the 
wiraa et open PLune other cases above mentioned, but it 
Mohl. may be observed more or less in all cells 
where active changes are going on; thus 
it may be especially seen in those of the pith of Hoya carnosa 
(fig. 79). A section of one of these cells gives an appearance as 
if the walls had been formed by concentric layers of cellulose 
with branching capillary tubes or canals stretching from the 
cavity of the cell to its periphery (fig. 79). The irregular ringed 
appearance is due to the difference in the degree of hydration, 
such as was seen in the case of the starch granule (page 32) ; 
while the canals are true passages, which have been caused by 
the passage of the sap during the life of the cell preventing the 
deposition of cellulose. In these cells the membrane has been 
still further changed by the conversion of the cellulose into 
lignin. It is to these two conditions that the firmness of the 
wood of plants and the hardness of the stones of many fruits 
are due, and hence the name of Sclerenchymatous (from a Greek 
word signifying hardness) has been given to such cells. 
Pitted or Dotted Cells.—When the cell-wall has thus become 
thickened it commonly presents (instead of the smooth and 
homogeneous appearance as is the case, as we have seen, when 
it is in a young condition) a greater or less number of dots or 
slits of various kinds (figs. 80 and 81, e, e). These dots and 
slits were formerly considered as actual openings in the walls of 
the cells, and hence such cells were called porous cells ; but, when 
carefully examined, it may be readily discovered that these 
markings are caused by canals which run from the cavity of the 
cell to the inside of its wall, and are closed (always at least in 
Fic. 79. 
