494 Minnesota Plant Life, 



both plants and animals cell structure is almost universal and 

 both kinds of creatures are said to be composed of cells. It 

 must be observed, however, that the cells are not the funda- 

 mental units of structure, but are themselves individualized 

 areas arising through adaptation of the living substance to its 

 surroundings. 



The growth of cells. Certain distinctions appear to exist 

 between the growth of cells and the growth of living substance. 

 When the living substance grows there is an actual increase 

 in its mass. A cell, however, may grow by distending itself 

 with water and there need be no absolute increase in its liv- 

 ing substance. A typical cell shows a delicate wall surround- 

 ing the living substance, and a portion of the latter is differ- 

 entiated from the rest as a complex body termed the nucleus. 

 Within the meshes of the living substance cell sap occurs. A 

 normal condition of such a cell is the plump, water-swollen, 

 elastic state known as turgor. If enough of the cell sap is lost 

 by evaporation through the delicate wall, the turgid condition 

 is no longer maintained and the cell becomes limp and flabby. 

 An organ composed of cells, as for example, a leaf, when the 

 cells have lost their turgidity through evaporation, becomes 

 wilted. In such a condition cell growth cannot go on, and 

 it may be stated categorically that for the growth of cells turgor 

 is a prerequisite. 



Maintenance of turgor. In order to understand how turgidity 

 of a cell is established and maintained, the common physical 

 phenomenon of liquid dialyzing through a membrane must be 

 thoroughly understood. The living substance of a cell, having 

 an avidity for water, will readily absorb it if it can be obtained. 

 Since, however, having adopted the cell-habit, each particle of 

 living substance is surrounded by a continuous membrane or cell 

 wall, the liquid, to reach the interior, must filter through this 

 membrane. It is a fact of observation that certain liquids or 

 solutions pass through membranes much more readily than do 

 others. Thus, if a bladder be filled with brine and placed in a 

 tub of fresh water it will soon become swollen, because the 

 fresh water passes inward more rapidly than the salt water 

 passes outward. If, however, a bladder, distended with fresh 

 water should be placed in a tub of brine the reverse action 



