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 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 

 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 

 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 



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. 



