104 PLANT PHYSIOLOGY 



helpful in the autumn to list the plants most susceptible to 

 injury and those that suffer least from frost. 



147. Growth. In the one-celled plants, or plants 

 made up of undifferentiated cells, growth is a function of 

 every cell. It enlarges up to a certain point and then 

 divides into two cells which enlarge and divide, etc. 

 In some cases the cell divides internally into many small 

 cells which enlarge until they reach the size of the parent 

 cell and repeat the process. The growth of a cell in- 

 volves a number of factors. Among these are the in- 

 crease in the amount of cytoplasm and sometimes a great 

 increase in the amount of cell sap, also the enlargement 

 of the cell wall in area and frequently also in thickness. 

 These cells are meristematic in many features. In such 

 plants we can hardly dissociate growth from reproduction. 



148. In the more complex plants we find some parts 

 that are the seat of the growth, the growing points and 

 adjacent region and cambium layers, while the rest of the 

 plant practically ceases to grow. The reproductive 

 functions are carried on by special parts of the plant 

 which have nothing to do with its ordinary growth. 

 The growth in such plants takes place still by the 

 process of cell growth and division, but we find that these 

 differ considerably from the case in one-celled plants. 

 Thus near the tips of the growing points the cells in- 

 crease their cytoplasm and cell wall area so as to become 

 perhaps twice as large, and then divide and form new cells 

 as is the case in one-celled plants except that the cells 

 remain attached. Gradually, however, some of these 

 cells that by the formation of new cells have come to lie 

 further from the tip increase more and more in size 

 and are not so active in their division. This increase in 

 size takes place largely by an increase in size of the 

 vacuoles so that the cells contain proportionally less and 

 less cytoplasm, although probably the amount of cyto- 



