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 

 Tie^fr""''^"'' 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 as a 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 



