312 VEGETABLE PHYSIOLOGY 



there is generally least resistance on the side of the free 

 apex of the organ, and hence an increased protrusion of 

 the latter results. Whatever may be the distribution of 

 such pressure the growth of the cell will be greatest in the 

 line of least resistance. If any internal cause should give 

 rise to differences in the uniformity of hydrostatic pressure 

 in all directions, the growth will be most extensive in the 

 line of the greatest. In the second place the extensibility 

 of the cell-wall may be locally modified by the protoplasm, 

 so that a uniform internal hydrostatic pressure may affect 

 one part more than another, and the growth consequently 

 will become irregular, giving rise in many cases to cells of 

 curious form. 



If we consider the behaviour of a growing organ in the 

 light of these facts, we shall see that, like the cell, it must 

 show a grand period of growth. If we take the case of a 

 root, in which the changes can be traced most easily on 

 account of the simplicity of its structure, we find that just 

 behind the apex the cells are all in active division. Growth 

 is small and consists mainly in an increase of the quantity 

 of protoplasm, for the cells divide again as soon as they 

 have reached a certain size. As new cells are continually 

 formed in the merismatic mass, those which are farthest 

 from the apex gradually cease to divide and a different 

 process of growth takes place in them, which is associated 

 more particularly with the formation of the vacuoles and 

 consequently with the establishment of considerable hydro- 

 static pressure, thus causing the bulk of the cells to be greatly 

 enlarged, as we have described. Hence it is here that 

 the actual extension in length of the root goes on, and the 

 cells reach the maximum point of the grand period. They 

 then gradually lose the power of growth, the oldest ones or 

 those farthest from the apex parting with it first, and they 

 pass gradually over into the condition of the permanent 

 tissue (fig. 135). In this way each zone of the root which 

 may be distinguished goes through a grand period of 

 growth. At first when the cells are merismatic, growth is 



