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 heen 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 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 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. (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 



