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 proc- 

 esses in seed germination. In gerihi- 

 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 

 process and only occurs in connec- 

 tion with protoplasm. 



Osmosis. — Osmosis may be de- 

 fined as that kind of diffusion by 

 which liquids pass through mem- 

 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 

 havmg 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 m the bladder be placed in the jar, the water slowly 

 passes out of the bladder. Thus the water passes from the weaker 



Fig. 109. — Experiment dem- 

 onstrating osmosis. The pig's 

 bladder was filled with a sugar 

 solution and then the tube was 

 attached. The water from the 

 jar was drawn into the bladder 

 and the solution in the bladder 

 forced up the tube. 



