314 VEGETABLE PHYSIOLOGY 
it is associated with the formation of new protoplasts. We 
have already mentioned that in the sporophytes of all 
the higher plants there exist certain regions in which 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 orginal dimensions and then divides again. These 
regions have been called growing points (fig. 183); they 
may be apical or intercalary. In such stems and roots as 
erow 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 hydrostatic pressure in 
the cell. This condition we have already considered in an 
earlier chapter, in which we discussed the relation of proto- 
plasm 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 turgescence, 
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 
