308 VEGETABLE PHYSIOLOGY 
the cells are merismatic—that is, which have the power of 
cell-multiplication by means of division. In such regions, 
when a cell has reached a certain size, which varies with 
the individual, it divides into two, each of which increases 
to the original dimensions and then divides again. These 
regions have been called growing points (fig. 133); they 
may be apical or intercalary. In such stems and roots as 
grow in thickness there are other growing regions, which 
consist of cylindrical sheaths known as cambiwm layers or 
phellogens. By the multiplication of the protoplasts in 
these merismatic areas the’ substance of the plant is 
increased. In other words, as these growing regions 
consist of cells, the growth of the entire organ or plant will 
depend on the behaviour of the cells or protoplasts of which 
its merismatic tissues are composed. 
The growth of such a cell will be found to depend 
mainly upon five conditions: (1) There must be a supply of 
nutritive or plastic materials, at the expense of which the 
increase of its protoplasm can take place, and which supply 
the needed potential energy. (2) There must be a supply 
of water to such an extent as to set up a certain hydro- 
static pressure in the cell. This condition we have already 
considered in an earlier chapter, in which we discussed 
the relation of protoplasm to water. (8) The supply of 
water must be associated with the formation of osmotic 
substances in the cell, or it cannot be made to enter it. 
In the absence of the tugescence, which will be the result 
of the last two conditions, no growth is possible for reasons 
that will presently appear. (4) The cell must have a 
certain temperature, for the activity of a protoplast is 
only possible within particular limits, which differ in the 
cases of different plants. (5) There must be a supply of 
oxygen to the growing cell, for, as we have seen, the 
protoplast is dependent upon this gas for the performance 
of its vital functions, and particularly for the liberation of 
the energy which is demanded in the constructive processes. 
This is evident also from the consideration that the growth 
