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- 
