CHAPTER XX. 



GROWTH. 



I. Growth. — We have seen in a previous chapter that at the 

 apex of a stem or root of an ordinary green plant, there is usually 

 z. formative region where the component small cells are in a state 

 of division, and new cells are being manufactured. Immedi- 

 ately behind this is a longer or shorter portion which may be 

 designated the growing region of the stem or root. Here the 

 cells are found to be turgid, and in consequence of the pressure 

 within them have increased in size, and at the same time many 

 of them have become changed in form. 



These changes of size and form, owing to increased turgidity 

 do not, however, necessarily constitute growth, although they 

 are always associated with growth. Cells which are growing not 

 only become distended by the osmotic pressure within the 

 vacuoles, but also undergo a permanent change in size, form 

 and structure, in consequence of the deposition of substances 

 in their cell-walls and other parts; on withdrawing water from 

 such cells the original state in which they existed when first 

 produced in the formative region is not again reproduced by 

 such a proceeding. Moreover, since the growth of a cell cannot 

 go on without increased turgidity, and as this involves an 

 addition of water to the vacuole of the cell, there is always an 

 increase in the total weight of the cell when growth is proceed- 

 ing : however, on account of the loss of substance by respiration, 

 there may be a decrease in its dry weight if such loss is not 

 compensated by anabolic nutritive processes. 



What is true of a single growing cell is also true in the case 



