Minnesota Plant Life. 495 



would take place and the bladder would become limp. If, fur- 

 ther, syrup were placed in the bladder and brine around it, the 

 first instance would be repeated and the bladder would become 

 swollen. Such observations indicate that syrup passes more 

 slowly through the membrane than does brine. In general it 

 may be said that solutions of relatively complex cliemical sub- 

 stances move through membranes more slowly than do solu- 

 tions of relatively simple substances. The phenomenon of 

 dialysis through a membrane is termed osmose. 



Living cells, since they are provided with cell sap and mem- 

 branes, are in a condition to maintain osmotic relations either 

 with neighboring cells or with any moisture-containing medium 

 that surrounds them. The presence or absence of osmotically 

 active compounds dissolved in the cell sap has much to do 

 with the regulation of turgor, as it has also with the conduc- 

 tion of food-solutions from one part of an organ to another. 

 Thus, not only are the various cells of a root-surface kept turgid 

 by the maintenance of an inward flow of liquid from the soil, 

 but compounds such as soluble salts of nitrogen or of metals 

 are transported- — along special paths in the higher plants — until 

 they reach, perhaps, a starch-making and proteid-making organ 

 such as the leaf. Not only is this upward flow of crude food 

 solutions normal for the higher green plants, but simultane- 

 ously there is, of refined food-products, such as sugar and pep- 

 tones, a downward flow from the leaves to other parts of the 

 plant, even to the most distant rootlets. 



A growing cell may then, upon the whole, be regarded as a 

 self-inflating bladder, which, through the activity of the living 

 substance, absorbs water from its surroundings. It does this 

 by the aid of osmotically active compounds produced bv the 

 living substance and dissolved in the cell sap. In highly organ- 

 ized plants, such as the ordinary trees, other factors besides 

 osmose enter into the ascent of the sap, but need not here be dis- 

 cussed. 



Growth of tissues and organs. Tissues, that is, structural 

 cell-aggregates, and organs, that is, physiological tissue-aggre- 

 gates, grow by the growth of their component cells. There 

 may be, however, cell-growth without, in a strict sense, growth 

 of the tissue or organ. For example, a nasturtium leaf-stalk 



