30 THE TISSUES OF PLANTS 
in meristem but are still usually thin, although in certain 
modifications, e.g. the parenchyma occurring in wood 
and sometimes that in the pith of woody twigs, the walls 
may be considerably thickened. In composition the 
wall is usually a form of cellulose except where thicken- 
ing has begun in which case the walls are often lignified. 
A large vacuole occupies the center of the cell and leaves 
the cytoplasm as a thin parietal layer (i.e. lining the wall) 
although there are often cytoplasmic strands running 
across the cell from one side to the other through the 
vacuole. The nucleus is generally imbedded in the 
parietal cytoplasm and appears relatively small owing 
to the great increase in size of the cell in its development 
from meristem, unaccompanied by a corresponding 
increase in the size of the nucleus. The chloroplasts are 
well developed in those parenchyma cells exposed to the 
light (except of course in plants devoid of chlorophyll). 
Very generally at the angles of contact of three or more 
parenchyma cells the middle lamella is ruptured or dis- 
solved and the corner of each cell be- 
comes rounded off leaving a space 
which becomes filled with air, a so- 
called intercellular space, these form- 
ing a continuous aerating system 
throughout the living parts of the 
Fic. an plant. In some parts of a plant, 
as in the pith, the parenchyma cells 
die early and the cell contents disappear, being re- 
placed by air. Probably this occurs by the absorption 
of the protoplasm by the adjacent cells. 
Laboratory Studies. (a) For undifferentiated cells examine 
the one-celled green slime plants (Protococcus)found asa green 
coating on the north side of trees or walls and the many-celled 
pond scums (such as Spirogyra or Zygnema) or one of the sim- 
ple filamentous blue-green algae (as Oscillatoria) which often 
