Meristems 



81 



where the initials are in tiers, this is done by radial divisions much like 

 the tangential ones. More commonly, however, the initial cell divides by 

 an obliquely radial (pseudotransverse) wall, and the daughter cells move 

 past one another until the two initials reach normal length and lie side 

 by side tangentially. The number of such divisions is usually more than 

 enough to make the number of initials conform to the enlarging circum- 

 ference of the cambium (Bannan, 1953). In many cases the daughter 

 cells fail to maintain themselves, and the rows of cells coming from 

 them are pinched out and gradually disappear (Bannan and Bayly, 

 1956). This process is so regulated, however, that the normal tangential 

 diameter of the mature cells is essentially maintained. The length of the 

 vascular elements originating from the cambium is also regulated to an 

 approximately constant size. 



Fig. 4-19. Early stages in the division of a cam- 

 bial cell, near the middle of a long initial. The 

 nucleus has divided, and the cell plate is being 

 formed by the fibers of the kinoplasmosome 

 (fc). (From I. W. Bailey.) 



The change in relative position of these enlarging cambial daughter 

 cells involves a problem in cellular readjustments that is of importance 

 morphogenetically. In 1886 Krabbe published a monograph on what he 

 termed "sliding growth," presenting evidence that during the differentia- 

 tion of tissues there was a good deal of change in intercellular position 

 brought about by the slipping or gliding of one cell past another. This 

 is common in animal embryology, where cells are more plastic and 

 often migrate for some distance, and is responsible for many of the 

 changes that take place in the development of these organisms. Its oc- 

 currence in plants might therefore be expected and was generally ac- 

 cepted as true for some time. Krabbe was supported in his position by 

 some other botanists, notably Neeff ( 1914 ) and Grossenbacher ( 1914 ) . 



Priestley ( 1930 ) criticized Krabbe's conclusion and believed that all 

 intercellular changes were brought about by what he termed "sym- 



