THE ANGIOSPERMAE : STEMS 853 



the place, direction, rate and time of growth in all parts and therefore control 

 the development of the specific form of the plant. 



The extent of and the relationships between the tissue zones at the apex 

 reflect the direction and distribution of growth, which we can detect by the 

 direction of the cell walls. Divisions are more frequent on the sides and at 

 the base of the corpus than they are in the centre, and the sides may con- 

 tribute to the peripheral tissues, while the base builds only the pith. The 

 morphogenetic functions of the zones are, however, undoubtedly variable. 



Below the corpus there is a well-marked transitional zone, called the 

 rib meristem, where growth is mainly longitudinal and each cell divides 

 transversely several times in succession, producing short vertical files of 

 cells, surrounded by older, thicker walls and traversed by young walls. 

 This tissue passes over gradually into typical parenchymatous pith by 

 the equalization of the wall thickness and the expansion of the cells and 

 readjustment of their relative positions. Elongation seems to be the chief 

 change in all the submeristematic cells, conditioned partly by the resistance 

 to transverse expansion exercised by the slower growing surface tissues and 

 partly by the pattern of the thread-like molecules in the cell walls, which 

 grow by longitudinal additions. Cells destined for vascular elements often 

 elongate very precociously, even in the meristem, as if their cells were 

 abnormally plastic, an efi'ect attributed by some to the diffusion of auxin from 

 the meristem. As we shall see in Volume III this diffusion is known to 

 occur, while the effect of auxins in increasing the plasticity of cell walls is 

 well known. Variation in the concentration of auxins at different points 

 may therefore have important results in controlling the differentiation of 

 tissues and the theory demands further investigation. 



All this elongation of cells affects the spacing of the rudimentary leaves. 

 They are pushed apart by the lengthening of the internodes and no longer 

 overlap their neighbours, but assume the spacing and position of mature 

 leaves. The length of an internode depends much more on the number of 

 its cells than upon variations in cell size, and it is a general rule of growth 

 that the longer active cell-division persists the larger is the resulting organ. 



Differentiation of Tissues 



The apical region of the stem is essentially a dynamic and highly plastic 

 organization, and the developments that arise from it follow paths which 

 are highly variable between species. During the past hundred years it has 

 been the aim of research to discover the laws of this development and to 

 bring some order into our ideas respecting it, but these efforts have not 

 been highly successful. A superfluity of descriptive terms have been coined 

 and they have been used in different senses by so many different authors that 

 confusion has abounded instead of clarity. 



The attempt to apply rigid definitions to the protean appearances in the 

 differentiation of tissues has led, for example, to such absurdities as the 

 inclusion of the protoxylem, in some cases, among " secondary " structures. 



