464 



BOTANY 



colored solution, the latter may pass readily through the cell-wall, 

 without being able to penetrate through the plasma-membranes into 

 the cell-sap, so long as the cell is alive. 



The cell-wall, being much more coherent, as well as more permea- 

 ble, than the plasma-membranes, allows a high degree of osmotic 

 pressure within the cell, without being ruptured. This would be 

 impossible in a cell provided only with a plasma-membrane. The 



tensility and permeability of the 

 cell-wall are of great importance 

 in the absorption of solutions, as 

 well as in giving firmness to the 

 masses of cellular tissue. The 

 mechanism of absorption is dios- 

 motic. If the density of the solu- 

 'tions within the cell is greater 

 than outside, as is usually the 

 case, there will be a movement 

 inward, and the food solutions out- 

 side will be carried into the cell, 

 an d it becomes turgid and larger. 

 If the cell is brought into a denser 

 medium (e.g. when a cell is placed 

 in a strong sugar or saline solu- 

 tion), part of the water is with- 

 drawn from the cell, and it loses 

 its turgidity, becoming flaccid and 

 smaller. Should the density of 

 the solution on both sides of the 

 cell-wall be the same, equilibrium 

 is established and all movement 

 ceases. But as the physical and 

 FIG. 451. A, B, seedlings of Mustard, chemical changes within the active 

 In B , the root-hairs are freed from cdl are constant l y disturbing this 

 the adherent soil-particles, covering . * , 



the root in A C, root-hair of wheat, equilibrium, and similar changes 

 much magnified, showing the ad- are occurring in all the cells, the 

 hesion of the soil-particles. (After movem ents of fluids from cell to 

 SACHS.) . 



cell in the active tissues are prac- 

 tically continuous, and, in growing parts, the young cells are in 

 a condition of perpetual turgor. 



In unicellular plants, or plants like the simpler Algae, composed 

 of but a few similar cells, every cell can absorb water containing the 

 dissolved food substances, whether solids or gases. In the higher 

 plants, especially terrestrial plants, there are special absorptive 

 organs, roots, and root-hairs developed, whose principal function is 

 the imbibition of food solutions from the earth. The cells directly 



