DYNAMICS OF THE GROWTH PROCESS 551 



tinuously by proliferation of the fundamental tissues of which these organs 

 are composed, but also increase in diameter as a result of cambial activity. 

 The tissues developed as a result of cambial activity, together with those 

 developed by certain special tj'pes of meristcms such as cork cambiums, are 

 called secondary tissues. 



Some increase in diameter may occur, however, even in stems and roots 

 which do not possess a cambium. Increase in the girth of a young stem as 

 a result of primary growth may continue for some time after increase in 

 length of that region of the stem has ceased, due to a slow continuance of 

 cell division and enlargement in some of the tissues, particularly those near 

 the periphery. In species in which the primary tissues constitute the entire 

 body of the plant this is the sole mode of growth in diameter. 



Dynamics of the Growth Process. — Growth almost invariably involves 

 not only a progressive increase in the dry weight of the growing region, 

 but a series of differentiation phenomena which become increasingly complex 

 during the growth process. Physiological differentiation of the protoplasm 

 begins before cell division or any other observable manifestations of growth 

 can be detected. Such differentiation undoubtedly continues throughout most 

 or all of the growth process but is sooner or later accompanied by other kinds 

 of differentiation such as size and shape differentiation of cells, structural 

 and chemical differentiation of cell walls, etc. The organized tissue systems 

 of mature plant organs are developed as a result of the coordinated differentia- 

 tion of cells during the growth process. 



The dynamics of growth will be described principally as exemplified by 

 the apical stem meristem of a dicot. Under a microscope examination of a 

 section cut longitudinally through a stem tip reveals three easily distinguishable 

 but intergrading regions usually termed ( i ) the region of cell division which 

 includes the so-called promeristem, (2) the region of cell enlargement, and 

 (3) the region of cell maturation (Fig. 117). These regions are usually 

 considered to correspond to the three principal morphological phases of growth. 



The developmental stages through which the cells at any level of the 

 stem axis pass can thus be found at any given moment in successive levels 

 of the axis, each in turn farther away from the apex. The appearance of the 

 cells at progressively lower levels back of the stem terminus indicates the 

 procession of modifications which the apical cells of that stem tip would 

 normally have undergone during its elongation, had their usual destiny not 

 been interrupted by preparing the tissue for miscroscopic examination. 



Such sections represent only the appearance of the cells through one 

 longitudinal plane of a growing stem tip at one particular instant in its 

 history. They can no more afford any adequate representation of the kalei- 



