DIV. ii PHYSIOLOGY 285 



The meristematic cells of the growing point contain considerable 

 amounts of imbibed water in the wall and protoplasm. As absorp- 

 tion of water from without continues, a distinction becomes evident 

 between the fully-saturated protoplasm and the vacuoles filled with 

 a watery solution ; this leads ultimately to the single large central 

 vacuole or sap-cavity surrounded by the peripheral layer or sac of 

 protoplasm (cf. p. 12, Fig. 3). It has been already seen (p. 225) 

 that the vacuole is the seat of osmotic forces ; the turgidity of the 

 cell is essential to the growth in surface of the cell wall. 



FIG. 258. Leaf of Begonia used as a cutting and bearing regenerative shoots. (After STOPPEL.) 



Cells in which the turgescence has been destroyed by plasmolysis (p. 226) 

 exhibit no further growth, and it might be concluded from this that the mechanical 

 distension of the wall assists or renders possible its growth. No clear corre- 

 spondence between distension and growth can, however, be assumed to exist. More- 

 over, the pressure of turgescence cannot be replaced by mere mechanical stretch- 

 ing of the wall. The protoplasm plays the main part in the growth in surface 

 of the cell wall, and in connection with this it can be understood how the walls 

 of cells that are only slightly distended may grow rapidly. 



Regarding the processes in the growth of cell wall which are termed apposition 

 and intussusception, svhat is necessary has been stated on p. 35. In growth in 

 surface due to plastic stretching without addition of material, followed by the 

 addition of new layers to the wall, the stretching due to turgor appears as a 

 natural preliminary to the growth. In the case of growth by intussusception the 

 turgor pressure appears less necessary. 



With the increased absorption of water following on the growth of the wall 



