GEOWTH 309 



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. 135) ; 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 cambium 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. (3) 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 

 the energy which is demanded in the constructive processes. 

 This is evident also from the consideration that the growth 



