116 CELLS AND TISSUES 
There are many ways in which cell walls are modified as will be 
seen in the study of tissues. 
Processes Involved in Cell Activity. — The chief of cell activi- 
ties is growth which will be discussed in connection with the differ- 
ent plant organs. But growth, besides being much under the 
influence of external conditions, such as temperature and light, 
depends upon metabolism — the proc- 
ess by which materials are changed 
into forms which have to do with 
growth. In connection with growth, 
metabolism, and other physiological 
processes of the cell, osmosis and res- 
piration are involved, both of which 
were shown to be important prec- 
esses in seed germination. In germi- 
nation and other physiological proc- 
esses they are important because of 
their connection with the other proc- 
esses of cells. Osmosis is a physical 
process which occurs wherever two 
liquids differing in concentration are 
separated by a membrane which they 
wet, and hence is not a cell activity 
except in so far as the protoplasm 
controls it when occurring in connec- 
tion with the cell. Respiration, on 
: , ; the other hand, is a physiological 
onstrating osmosis. The pig’s : 
Bladder ous Bled with a cunae DOC and only occurs in connec- 
solution and then the tube was tion with protoplasm. 
attached. The water from the Osmosis. — Osmosis may be de- 
jar was drawn into the bladder fined as that kind of diffusion by 
and the solution in the bladder whieh liquids pass through mem- 
Joveed up the tube. branes and its principles can be best 
understood by the study of an illustration as shown in Figure 
109. Thus if a pig’s bladder, filled with a sugar solution and 
having a long glass tube fastened in its neck, is submerged in 
a jar of water, water will pass in and force the solution up the 
glass tube. If, on the other hand, a sugar or salt solution stronger 
than the one in the bladder be placed in the jar, the water slowly 
passes out of the bladder. Thus the water passes from the weaker 
Fie. 109.— Experiment dem- 
