CELLULAR TISSUE. 35 



portion of its liquid contents to a contiguous cell, which may be 

 charged with contents of greater density than its own. From the 

 nature of the process of assimilation and other operations carried 

 on in the interior of cells, they must always contain a denser fluid 

 than the water in which aquatic plants live, or which is presented 

 to the roots or other parts of the surface of terrestrial plants. This, 

 with the gaseous and other matters it holds in solution, the vegeta- 

 ble must constantly tend to imbibe by endosmosis. In virtue of 

 the same law, as will hereafter be explained, not only is the crude 

 food imbibed by the roots, but transferred from cell to cell to the 

 place where assimilation is principally effected or growth is going 

 on. In addition to this simpler process, animals, even of the 

 lowest grades, have a proper circulation through vessels. There 

 is no such circulation in plants. 



38. Growth of Cell-membrane interstitially, By appropriating the 



assimilated matter it contains or imbibes, the young cell increases 

 rapidly in size; its wall is extended equally on every side (unless 

 something interferes with its expansion in particular directions), so 

 that a larger space is surrounded. Meanwhile, instead of becorn- 



stronger, with a force proportioned to the difference in density (endosmosis) ; 

 but at the same time, a smaller portion of the denser liquid will flow out into 

 the weaker (exosmosis). Thus, if the lower end of an open tube, closed with 

 a thin membrane, such as a piece of moistened bladder, be introduced into a 

 vessel of pure water, and a solution of sugar in water be poured into the tube, 

 the water from the vessel will shortly be found to pass into the tube, so that 

 the column of liquid it contains will increase in height to an extent propor- 

 tionate to the strength of the solution. At the same time, the water in the 

 vessel will become slightly sweet; showing that a small quantity of syrup has 

 passed through the pores of the membrane into the water without, while a 

 much'larger portion of water has entered the tube. The water will continue 

 to enter the tube, and a small portion of syrup to leave it, until the solution is 

 reduced to the same strength as the liquid without. If a solution of gum, salt, 

 or any other substance, be employed instead of sugar, the same result will 

 take place. If the same solution be employed both in the vessel and the tube, 

 no transference or change will be observed. But if either be rendered strong- 

 er than the other, a circulation will be established, and the stronger solution 

 will increase in quantity until the two attain the same density. If two differ- 

 ent solutions be employed, as, for instance, sugar or gum within the tube, and 

 potash or soda without, a circulation will in like manner take place, the pre- 

 ponderance being towards the denser fluid, and in a degree exactly propor- 

 tionate to the difference in density. Instead of animal membrane, any vegeta- 

 ble matter with fine pores, such as a thin piece of wood, or even a porous 

 mineral substance, may be substituted with the same result. 



